Explicitly enable the FSL_XGMAC_MDIO Kconfig option in order to have
MDIO access to internal and external PHYs.

Fixes: 71947923 ("dpaa2-eth: add MAC/PHY support through phylink")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Link: https://lore.kernel.org/r/20201119145106.712761-1-ciorneiioana@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>