net: dsa: bcm_sf2: Add missing OF_MDIO dependency

net: dsa: bcm_sf2: Add missing OF_MDIO dependency

bcm_sf2 does require the MDIO_BCM_UNIMAC driver which is now dependent
on OF_MDIO but also internally uses of_mdio.c provided routines which
are guarted with OF_MDIO.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 90eff9096c01 ("net: phy: Allow splitting MDIO bus/device support from PHYs")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>