net: phy: MDIO_BCM_UNIMAC should depend on OF_MDIO

The Broadcom MDIO UniMAC driver uses routines provided by of_mdio.c which is
guarded by CONFIG_OF_MDIO.

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