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_protocol_embed.h b/drivers/arm/rss/rss_comms_protocol_embed.h
index 6c2a7bd..609d099 100644
--- a/drivers/arm/rss/rss_comms_protocol_embed.h
+++ b/drivers/arm/rss/rss_comms_protocol_embed.h
@@ -30,13 +30,13 @@
 	psa_handle_t handle;
 	uint32_t ctrl_param; /* type, in_len, out_len */
 	uint16_t io_size[PSA_MAX_IOVEC];
-	uint8_t trailer[RSS_COMMS_PAYLOAD_MAX_SIZE];
+	uint8_t trailer[PLAT_RSS_COMMS_PAYLOAD_MAX_SIZE];
 };
 
 struct __packed rss_embed_reply_t {
 	int32_t return_val;
 	uint16_t out_size[PSA_MAX_IOVEC];
-	uint8_t trailer[RSS_COMMS_PAYLOAD_MAX_SIZE];
+	uint8_t trailer[PLAT_RSS_COMMS_PAYLOAD_MAX_SIZE];
 };
 
 psa_status_t rss_protocol_embed_serialize_msg(psa_handle_t handle,