net: phy: add USXGMII link partner ability constants

The constants are taken from the USXGMII Singleport Copper Interface
specification. The naming are based on the SGMII ones, but with an MDIO_
prefix.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>