fix(linux): Update ICSSM FAQ on how to address low performance

In case of AM335x and AM437, the TCP performance numbers tend
to be low, where the bottle neck is the CPU frequency and the
throughput attained in the expected one. Explain the same in the
FAQ of ICSSM doc and also provide the command which can be used
if the user wants to attain max possible throughput.

Signed-off-by: Meghana Malladi <m-malladi@ti.com>

Author: MeghanaMalladiTI

source/linux/Foundational_Components/PRU-ICSS/Linux_Drivers/PRU-ICSS_Ethernet.rst

@@ -687,6 +687,23 @@ Turning flooding on/off on switch ports
 .. rubric:: Frequently Asked Questions
    :name: frequently-asked-questions
 
+.. ifconfig:: CONFIG_part_variant in ('AM335X','AM437X')
+
+   .. rubric:: Low TCP performance throughput on PRU-ICSS Ethernet ports, how to improve?
+      :name: low-tcp-performance-throughput-how-to-improve
+
+   The CPUFreq governer should be set to performance for use-cases requiring high throughput.
+   Hence to attain maximum throughput, force the CPU to run at its maximum frequency and
+   prevent any frequency scaling using the following command:
+
+   .. code-block:: console
+
+      echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
+
+   This forces the CPU to run at its maximum frequency and should improve the TCP throughput.
+   On AM335x, TCP throughput with 1500 byte (default) packet size, before and after running the
+   command are 26Mbps and 76Mbps respectively.
+
 .. rubric:: Are the HSR or PRP protocols supported?
    :name: are-the-hsr-or-prp-protocols-supported