enetc: auto select PHYLIB and MDIO_DEVRES

FSL_ENETC_MDIO use symbols from PHYLIB (MDIO_BUS) and MDIO_DEVRES,
however there are no dependency specified in Kconfig

ERROR: modpost: "__mdiobus_register" [drivers/net/ethernet/freescale/enetc/fsl-enetc-mdio.ko] undefined!
ERROR: modpost: "mdiobus_unregister" [drivers/net/ethernet/freescale/enetc/fsl-enetc-mdio.ko] undefined!
ERROR: modpost: "devm_mdiobus_alloc_size" [drivers/net/ethernet/freescale/enetc/fsl-enetc-mdio.ko] undefined!

add depends on MDIO_DEVRES && MDIO_BUS

Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>