build(drtm): add DRTM support build option

Added DRTM support build option in the makefiles.
This build option will be used by the DRTM implementation
in the subsequent patches.

Signed-off-by: Manish V Badarkhe <manish.badarkhe@arm.com>
Signed-off-by: Lucian Paul-Trifu <lucian.paultrifu@gmail.com>
Change-Id: I15366f86b3ebd6ab2ebcb192753015d547cdddee