fix(rss): rename AP-RSS message size macro

Adding PLAT_* prefix to indicate that the
platform needs to provide this definition.

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: I0bd02be405fd8b1e625bd2b82647ebb2b58265fc
diff --git a/drivers/arm/rss/rss_comms.c b/drivers/arm/rss/rss_comms.c
index a1f5daa..8b5002f 100644
--- a/drivers/arm/rss/rss_comms.c
+++ b/drivers/arm/rss/rss_comms.c
@@ -42,11 +42,11 @@
 
 	comms_embed_msg_min_size = sizeof(struct serialized_rss_comms_header_t) +
 				   sizeof(struct rss_embed_msg_t) -
-				   RSS_COMMS_PAYLOAD_MAX_SIZE;
+				   PLAT_RSS_COMMS_PAYLOAD_MAX_SIZE;
 
 	comms_embed_reply_min_size = sizeof(struct serialized_rss_comms_header_t) +
 				     sizeof(struct rss_embed_reply_t) -
-				     RSS_COMMS_PAYLOAD_MAX_SIZE;
+				     PLAT_RSS_COMMS_PAYLOAD_MAX_SIZE;
 
 	/* Use embed if we can pack into one message and reply, else use
 	 * pointer_access. The underlying MHU transport protocol uses a