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>