feat(rmmd): verify FEAT_MEC present before calling plat hoook

Some platforms do not support FEAT_MEC. Hence, they do not provide
an interface to update the update of the key corresponding to a
MECID.

This patch adds a condition in order to verify FEAT_MEC is present
before calling the corresponding platform hook, thus preventing it
from being called when the platform does not support the feature.

Change-Id: Ib1eb9e42f475e27ec31529569e888b93b207148c
Signed-off-by: Juan Pablo Conde <juanpablo.conde@arm.com>
diff --git a/include/plat/common/platform.h b/include/plat/common/platform.h
index 708cbcd..faccf31 100644
--- a/include/plat/common/platform.h
+++ b/include/plat/common/platform.h
@@ -399,7 +399,7 @@
 size_t plat_rmmd_get_el3_rmm_shared_mem(uintptr_t *shared);
 int plat_rmmd_load_manifest(struct rmm_manifest *manifest);
 int plat_rmmd_mecid_key_update(uint16_t mecid);
-#endif
+#endif /* ENABLE_RME */
 
 /*******************************************************************************
  * Optional BL31 functions (may be overridden)