docs(drtm): add platform APIs for DRTM

Documented platform APIs for DRTM

Signed-off-by: Lucian Paul-Trifu <lucian.paultrifu@gmail.com>
Signed-off-by: Manish V Badarkhe <manish.badarkhe@arm.com>
Change-Id: I22749c26bbe7b3271705dd3db07e8597fce6225b
1 file changed