net: phy: add generic UniMAC MDIO bus driver

Add a generic UniMAC MDIO bus driver and its Device Tree binding, which
can be used by the BCMGENET driver as-is, and the upcoming Starfighter 2
Ethernet switch MDIO bus controller.

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