net: phy: Add phy loopback support in net phy framework

This patch add set_loopback in phy_driver, which is used by MAC
driver to enable or disable phy loopback. it also add a generic
genphy_loopback function, which use BMCR loopback bit to enable
or disable loopback.

Signed-off-by: Lin Yun Sheng <linyunsheng@huawei.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/marvell.c

@@ -2171,6 +2171,7 @@ static struct phy_driver marvell_drivers[] = {
 		.get_sset_count = marvell_get_sset_count,
 		.get_strings = marvell_get_strings,
 		.get_stats = marvell_get_stats,
+		.set_loopback = genphy_loopback,
 	},
 	{
 		.phy_id = MARVELL_PHY_ID_88E1540,