feat(power): overview: Note requirement of latest firmware on 12.0

ti-u-boot-2026.01 applies firewall patches over TF-A and OPTEE memory
region. This caused a regression in LPM, which was fixed in 12.0
firmware. Therefore, add a note stating that users should use latest U-Boot with
latest firmware, and attempting to use new U-Boot with old firmware will cause
LPM to fail.

Signed-off-by: Suhaas Joshi <s-joshi@ti.com>

Author: jsuhaas22

source/linux/Foundational_Components/Power_Management/pm_overview.rst

@@ -11,6 +11,12 @@ requires coordination between multiple software and hardware frameworks.
 Power Management techniques can broadly be classified into two categories:
 Dynamic Power Management and Static Power Management.
 
+.. note::
+
+   With ti-u-boot-2026.01, LPM works only if ti-linux-firmware is 12.0 or
+   newer. On older firmware images and new U-Boot, LPM fails and the device
+   requires a reboot.
+
 .. rubric:: Dynamic Power Management
 
 Dynamic Power Management techniques reduce the active power