feat(smccc): add vendor specific el3 id

Add vendor specific el3 function id and update docs for the same.

SMCCC Documentation reference:
https://developer.arm.com/documentation/den0028/latest

Change-Id: Ieeb63608ad74d7b764d7131d8a92ecf10053c50d
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
2 files changed