feat(drtm): add standard DRTM service

Added a dummy DRTM setup function and also, introduced DRTM SMCs
handling as per DRTM spec [1]. Few basic SMCs are handled in this
change such as ARM_DRTM_SVC_VERSION and ARM_DRTM_SVC_FEATURES
that returns DRTM version and functions ids supported respectively,
and others are dummy for now.

[1]: https://developer.arm.com/documentation/den0113/latest



Signed-off-by: Manish V Badarkhe <manish.badarkhe@arm.com>
Signed-off-by: Lucian Paul-Trifu <lucian.paultrifu@gmail.com>
Change-Id: I8c7afe920c78e064cbab2298f59e6837c70ba8ff