fix(rmmd): add missing padding to RMM Boot Manifest and initialize it

This patch also:
     * Enforces the check of RES0 fields on EL3-RMM boot interface
       and manifest
     * Fixes a couple of nits on the EL3-RMM Boot Interface
       documentation.

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: Idb9e38f9fcda2ba0655646a1e2c4fdbabd5cdc40
diff --git a/plat/arm/board/fvp/fvp_common.c b/plat/arm/board/fvp/fvp_common.c
index f8463f1..f5d9940 100644
--- a/plat/arm/board/fvp/fvp_common.c
+++ b/plat/arm/board/fvp/fvp_common.c
@@ -536,6 +536,7 @@
 	assert(manifest != NULL);
 
 	manifest->version = RMMD_MANIFEST_VERSION;
+	manifest->padding = 0U; /* RES0 */
 	manifest->plat_data = (uintptr_t)NULL;
 
 	return 0;