feat(el3-runtime): handle traps for IMPDEF registers accesses

This patch introduces support to handle traps from lower ELs for
IMPDEF system register accesses. The actual support is left to the
platforms to implement.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: I623d5c432b4ce4328b68f238c15b1c83df97c1e5
diff --git a/Makefile b/Makefile
index 98e448f..d472054 100644
--- a/Makefile
+++ b/Makefile
@@ -1204,6 +1204,7 @@
         TWED_DELAY \
         ENABLE_FEAT_TWED \
         SVE_VECTOR_LEN \
+	IMPDEF_SYSREG_TRAP \
 )))
 
 ifdef KEY_SIZE
@@ -1333,6 +1334,7 @@
         TWED_DELAY \
         ENABLE_FEAT_TWED \
 	CONDITIONAL_CMO \
+	IMPDEF_SYSREG_TRAP \
 )))
 
 ifeq (${SANITIZE_UB},trap)