FF-A: Change name of direct message wrappers

Changed the name of 'ffa_msg_send_receiver' and 'ffa_msg_send_sender'
to 'ffa_receiver' and 'ffa_sender'. The intent is to make it semantically
more generic, so they can be used in other contexts other than
FF-A direct messaging calls. E.g. Notifications interfaces.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I0c7e57f5b7646b3d19f6822370ccf609bcec7531
diff --git a/main.c b/main.c
index 22082d9..6e859cf 100644
--- a/main.c
+++ b/main.c
@@ -405,12 +405,12 @@
 
 		/* Response available. */
 		case FFA_MSG_SEND_32:
-			if (ffa_msg_send_receiver(ret) == PRIMARY_VM_ID) {
+			if (ffa_receiver(ret) == PRIMARY_VM_ID) {
 				hf_handle_message(vcpu->vm,
 						  ffa_msg_send_size(ret),
 						  page_address(hf_recv_page));
 			} else {
-				hf_deliver_message(ffa_msg_send_receiver(ret));
+				hf_deliver_message(ffa_receiver(ret));
 			}
 			break;