feat(pm_cpuidle): add detailed documentation for AM62 standby mode

Add comprehensive documentation for CPUIdle on AM62X, AM62AX, and AM62PX
platforms explaining:

- Standby mode implementation through CPUIdle WFI state
- Integration between Linux CPUIdle framework and TF-A
- PSCI interaction via cpu_standby and validate_power_state hooks
- Source file references for both Linux and TF-A components
- Kernel configuration options and usage information

This documentation helps users understand that the TI "standby" mode is
automatically handled by the CPUIdle framework without requiring special
configuration.

Signed-off-by: Dhruva Gole <d-gole@ti.com>

Author: DhruvaG2000

source/linux/Foundational_Components/Power_Management/pm_cpuidle.rst

@@ -20,6 +20,113 @@ transition to the selected state.
     C-state. This state gets enabled by default in the CPUIdle driver without
     requiring any additional DT configuration.
 
+    .. rubric:: Standby Mode
+
+    On |__PART_FAMILY_DEVICE_NAMES__| platforms, "Standby" mode is implemented through the Linux CPUIdle
+    framework. Standby is the default low-power state that the system enters automatically when idle,
+    using the WFI (Wait For Interrupt) C-state. This power-saving mechanism activates whenever the
+    system is "just sitting idle" without requiring any special configuration or activation from
+    the user.
+
+    When your device appears inactive, it is actually cycling in and out of this standby state
+    many times per second, seamlessly managing power while remaining responsive.
+
+    .. rubric:: Standby Implementation Architecture
+
+    For |__PART_FAMILY_DEVICE_NAMES__|, the Standby implementation involves a multi-layer architecture:
+
+    1. **Linux CPUIdle Framework**:
+       - Determines when to place CPUs into idle states based on system activity
+       - Uses governors (like the menu governor) to predict how long CPUs will be idle
+       - Invokes platform-specific drivers to transition to appropriate C-states
+
+    2. **ARM Generic CPUIdle Driver**:
+       - Implements the Linux CPUIdle driver interface for ARM platforms
+       - Communicates with TF-A through PSCI SMC (Secure Monitor Call) interface
+
+    3. **PSCI Interface in Linux**:
+       - Provides a standardized method for the kernel to request power management operations
+       - Translates Linux-side requests into PSCI calls to TF-A
+
+    4. **TF-A PSCI Implementation**:
+       - Runs in secure world (EL3)
+       - Implements the PSCI specification
+       - Provides platform-specific handlers via the ``plat_psci_ops`` structure
+
+    When your system goes idle, the following sequence occurs:
+
+    - Linux idle loop detects no runnable tasks
+    - CPUIdle governor selects the appropriate C-state (WFI for these platforms)
+    - CPUIdle driver communicates with TF-A via the PSCI interface
+    - TF-A's PSCI implementation validates the requested power state through ``validate_power_state()``
+    - TF-A executes the ``cpu_standby()`` handler from ``plat_psci_ops``
+    - The actual WFI instruction is executed within the TF-A's standby handler
+    - On interrupt: CPU automatically exits WFI state
+    - Control returns to Linux
+
+    This automated process happens continuously during system operation, seamlessly
+    transitioning between active and idle states to save power whenever possible.
+
+    .. rubric:: Standby vs. Deep Sleep: Understanding the Difference
+
+    It's important to distinguish between the lightweight "standby" provided by CPUIdle and deeper sleep states:
+
+    * **CPUIdle Standby (WFI)**:
+      - Processor-level power saving only
+      - Very fast entry and exit (microseconds)
+      - Occurs automatically hundreds of times per second
+      - No user intervention required
+      - All peripherals remain operational
+      - Perfect for normal "idle" periods
+
+    * **Deep Sleep Modes**:
+      - System-wide power saving
+      - Slower entry and exit (milliseconds to seconds)
+      - Requires explicit software requests
+      - May require peripheral reconfiguration
+      - Suitable for extended inactive periods
+
+    .. rubric:: Source Location
+
+    **Linux Side**:
+
+    - :file:`drivers/cpuidle/cpuidle-arm.c` - ARM CPU idle driver
+    - :file:`drivers/cpuidle/dt_idle_states.c` - Device tree parsing for idle states
+    - :file:`drivers/cpuidle/cpuidle.c` - Core CPUIdle framework
+    - :file:`kernel/sched/idle.c` - Kernel idle loop implementation
+    - :file:`drivers/firmware/psci.c` - PSCI interface to TF-A
+
+    **TF-A Side** (not part of Linux kernel):
+
+    - :file:`plat/ti/k3/common/k3_psci.c` - PSCI implementation for K3 platforms
+
+    .. rubric:: Driver Usage
+
+    CPUIdle works automatically once enabled in the kernel, with no user intervention required.
+    The CPUIdle framework statistics can be accessed through the sysfs interface:
+
+    .. code-block:: console
+
+        # ls -l /sys/devices/system/cpu/cpu0/cpuidle/
+        drwxr-xr-x    2 root     root             0 Jan  1 00:01 state0
+
+        # ls -l /sys/devices/system/cpu/cpu0/cpuidle/state0/
+        -r--r--r--    1 root     root         4096 Jan  1 00:02 desc
+        -r--r--r--    1 root     root         4096 Jan  1 00:02 latency
+        -r--r--r--    1 root     root         4096 Jan  1 00:02 name
+        -r--r--r--    1 root     root         4096 Jan  1 00:02 power
+        -r--r--r--    1 root     root         4096 Jan  1 00:02 time
+        -r--r--r--    1 root     root         4096 Jan  1 00:02 usage
+
+    To view the current C-state statistics:
+
+    .. code-block:: console
+
+        # cat /sys/devices/system/cpu/cpu0/cpuidle/state0/name
+        WFI
+        # cat /sys/devices/system/cpu/cpu0/cpuidle/state0/usage
+        6245738
+
 .. ifconfig:: CONFIG_part_family in ('AM335X_family', 'AM437X_family')
 
     .. rubric:: Driver Features
@@ -77,7 +184,6 @@ transition to the selected state.
             -*-   Menu governor (for tickless system)
                     ARM CPU Idle Drivers  ----
 
-
     .. rubric:: DT Configuration
 
     .. code-block:: dts