refactor(ff-a): add payload getters

Add `ffa_partition_msg_payload` and `ffa_partition_msg_payload_const`
functions for accessing the the payload of the partition message.

These functions are necessary because the `.payload` field does not
necessary correspond to the `.offset` field.

Change-Id: I91246a084ccb68c02809b3e1afe85b07482bfb1b
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/test/hftest/service_common.c b/test/hftest/service_common.c
index 6ac9162..78f6400 100644
--- a/test/hftest/service_common.c
+++ b/test/hftest/service_common.c
@@ -287,7 +287,7 @@
 	struct fdt fdt;
 	const ffa_id_t own_id = hf_vm_get_id();
 	ffa_notifications_bitmap_t bitmap;
-	struct ffa_partition_msg *message;
+	const struct ffa_partition_msg *message;
 	uint32_t vcpu = get_current_vcpu_index();
 
 	ctx = hftest_get_context();
@@ -356,7 +356,8 @@
 		ASSERT_EQ(hf_interrupt_get(), HF_NOTIFICATION_PENDING_INTID);
 	}
 
-	memiter_init(&args, message->payload, message->header.size);
+	memiter_init(&args, ffa_partition_msg_payload_const(message),
+		     message->header.size);
 
 	/* Find service handler. */
 	service = find_service(&args);