net: dsa: mv88e6060: Support probing as an mdio device

Probing DSA devices as platform devices has been superseded by using
normal bus drivers. Add support for probing the mv88e6060 device as an
mdio device.

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