net: phy: Add 2000base-x, 2500base-x and rxaui modes

The mv88e6390 ports 9 and 10 supports some additional PHY modes. Add
these modes to the PHY core so they can be used in the binding.

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