feat(smccc): add vendor-specific el3 service

Add support for vendor-specific el3 service. SMCCC 1.5 introduces
support for vendor-specific EL3 monitor calls.

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

Change-Id: Id8bc43842eecdb7a8a2ec7f31a631e88fe4fe0b4
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
7 files changed