net: phy: add genphy_c45_loopback

Add generic code to enable C45 PHY loopback into the common phy-c45.c
file. This will allow C45 PHY drivers aceess this by setting
.set_loopback.

Suggested-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>