feat(fvp): add platform hooks for DRTM DMA protection

Added necessary platform hooks for DRTM DMA protection.
These calls will be used by the subsequent DRTM implementation
patches.
DRTM platform API declarations have been listed down in a
separate header file.

Signed-off-by: Manish V Badarkhe <manish.badarkhe@arm.com>
Signed-off-by: Lucian Paul-Trifu <lucian.paultrifu@gmail.com>
Change-Id: Ib9726d1d3570800241bde702ee7006a64f1739ec