ras/cmn: add documentation for cmn kfh handling

Add documentation that describes the RAS support in CMN Cyprus (CMN S3).
It also outlines the SW support added to enable CMN Kernel first
handling on RD-V3-Cfg1 platform. It describes the how to write AEST ACPI
table and also provides steps to perform pseudo fault registers
supported error injection.

Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
Change-Id: I6c57a408803f9c191daed5768c5289529bb143e8