feat(indirect message): add FFA_MSG_SEND2 support

FF-A v1.1 indirect message FFA_MSG_SEND2 support, allowing indirect
messaging between any endpoint (VM and SP).
When one of the endpoints is an SP, the SPMC is responsible for message
delivery to the receiver, potentially in the other world.
In order to allow cross world communication the SPMC is aware of VM
mailboxes and the Hypervisor forwards the FFA_MSG_SEND2 message to the
SPMC when the receiver is an SP.

Change-Id: Iba9b3d299b051ed3b105cb9e2b63e65fd4f662ce
Signed-off-by: Federico Recanati <federico.recanati@arm.com>
diff --git a/inc/vmapi/hf/ffa.h b/inc/vmapi/hf/ffa.h
index 43b9d30..3011105 100644
--- a/inc/vmapi/hf/ffa.h
+++ b/inc/vmapi/hf/ffa.h
@@ -372,6 +372,11 @@
 	return args.arg4;
 }
 
+static inline uint32_t ffa_msg_send2_flags(struct ffa_value args)
+{
+	return args.arg2;
+}
+
 static inline uint32_t ffa_partition_info_get_count(struct ffa_value args)
 {
 	return args.arg2;