net: bcmgenet: utilize generic Broadcom UniMAC MDIO controller driver

Update the GENET driver to register an UniMAC MDIO bus controller for
the GENET internal MDIO bus, update the platform data code to attach the
PHY to the correct MDIO bus controller.

The Device Tree portion of the code is mostly left unmodified since the
lookup/binding is done via phandles and Device Tree nodes which are much
more flexible in locating and binding PHYs to their respective MDIO bus
controllers.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>