feat(qemu): add plat_rmmd_mecid_key_update()

Add an implementation of the plat_rmmd_mecid_key_update() callback, that
updates the MEC keys associated with a MECID. Leave it empty for now,
since QEMU doesn't yet implement an MPE (Memory Protection Engine).

Change-Id: I2746f6024f28e4fd487726de9e43e14d8cad57a0
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
diff --git a/plat/qemu/common/qemu_common.c b/plat/qemu/common/qemu_common.c
index a88297d..4a79a90 100644
--- a/plat/qemu/common/qemu_common.c
+++ b/plat/qemu/common/qemu_common.c
@@ -400,6 +400,18 @@
 
 	return 0;
 }
+
+/*
+ * Update encryption key associated with @mecid.
+ */
+int plat_rmmd_mecid_key_update(uint16_t mecid)
+{
+	/*
+	 * QEMU does not provide an interface to change the encryption key
+	 * associated with MECID. Hence always return success.
+	 */
+	return 0;
+}
 #endif  /* ENABLE_RME */
 
 /**