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
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index d5383a1..fab6bf6 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -463,3 +463,6 @@
 
 # By default, disable the mocking of RSS provided services
 PLAT_RSS_NOT_SUPPORTED		:= 0
+
+# Dynamic Root of Trust for Measurement support
+DRTM_SUPPORT			:= 0