net: phy: marvell10g: Add PHY loopback support

Add support for PHY loopback for Marvell 88x2110 and Marvell 88x3310.

This allow user to perform PHY loopback test using ethtool selftest.
Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/marvell10g.c

@@ -781,6 +781,7 @@ static struct phy_driver mv3310_drivers[] = {
 		.get_tunable	= mv3310_get_tunable,
 		.set_tunable	= mv3310_set_tunable,
 		.remove		= mv3310_remove,
+		.set_loopback	= genphy_c45_loopback,
 	},
 	{
 		.phy_id		= MARVELL_PHY_ID_88E2110,
@@ -796,6 +797,7 @@ static struct phy_driver mv3310_drivers[] = {
 		.get_tunable	= mv3310_get_tunable,
 		.set_tunable	= mv3310_set_tunable,
 		.remove		= mv3310_remove,
+		.set_loopback	= genphy_c45_loopback,
 	},
 };