feat(rss): add new comms protocols

The current comms protocol (where arguments and return data is embedded
into the MHU message) is now protocol v0. Protocol v1 embeds pointers
into the message, and has the RSS retrieve the data via DMA.

Change-Id: I08d7f09c4eaea673769fde9eee194447a99f1b78
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/drivers/arm/mhu/mhu_wrapper_v2_x.c b/drivers/arm/mhu/mhu_wrapper_v2_x.c
index d8b7cfd..60de1d3 100644
--- a/drivers/arm/mhu/mhu_wrapper_v2_x.c
+++ b/drivers/arm/mhu/mhu_wrapper_v2_x.c
@@ -300,3 +300,13 @@
 
 	return MHU_ERR_NONE;
 }
+
+size_t mhu_get_max_message_size(void)
+{
+	struct mhu_v2_x_dev_t *dev = &MHU1_SEH_DEV;
+	uint32_t num_channels = mhu_v2_x_get_num_channel_implemented(dev);
+
+	assert(num_channels != 0);
+
+	return num_channels * sizeof(uint32_t);
+}