net: phy: mdio-unimac: add compatible strings for GENET MDIO bus

The Broadcom GENET Ethernet MAC has a Broadcom UniMAC MDIO bus
controller, allow the mdio-unimac driver to drive this MDIO bus, this
will allow us to get rid of some MDIO bus specific code in the BCMGENET
driver.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
net: bcmgenet: utilize UniMAC MDIO bus

Remove the Broadcom GENET MDIO code which duplicates the MDIO bus driver
in drivers/net/phy/mdio-unimac.c.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>