mdio: Add support for mdio drivers.

Not all devices on an MDIO bus are PHYs. Meaning not all MDIO drivers
are PHY drivers. Add support for generic MDIO drivers.

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>