net: phy: add interface mode PHY_INTERFACE_MODE_USXGMII

Add support for interface mode PHY_INTERFACE_MODE_USXGMII.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>