dsa: Add mdio device support to Marvell switches

Allow Marvell switches to be mdio devices. Currently the driver just
allocate the private structure and detects what device is on the
bus. Later patches will make them register with the DSA framework.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>