net: mvpp2: update the port documentation regarding the GoP

The Marvell PPv2 port structure stores the GoP id of a given port. This
information is specific to PPv2.2, but cannot be used by PPv2.1. Update
its comment to denote this specificity.
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/mvpp2/mvpp2.h

@@ -803,7 +803,7 @@ struct mvpp2_port {
 	u8 id;
 
 	/* Index of the port from the "group of ports" complex point
-	 * of view
+	 * of view. This is specific to PPv2.2.
 	 */
 	int gop_id;