net: macb: add description for SRTSM

Add description for SRTSM bit.
Signed-off-by: NClaudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: NNicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/macb.h

@@ -243,7 +243,7 @@
 #define MACB_NCR_TPF_SIZE	1
 #define MACB_TZQ_OFFSET		12 /* Transmit zero quantum pause frame */
 #define MACB_TZQ_SIZE		1
-#define MACB_SRTSM_OFFSET	15
+#define MACB_SRTSM_OFFSET	15 /* Store Receive Timestamp to Memory */
 #define MACB_OSSMODE_OFFSET 24 /* Enable One Step Synchro Mode */
 #define MACB_OSSMODE_SIZE	1