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>

drivers/net/ethernet/freescale/enetc/Kconfig

@@ -27,7 +27,7 @@ config FSL_ENETC_VF
 
 config FSL_ENETC_MDIO
 	tristate "ENETC MDIO driver"
-	depends on PCI
+	depends on PCI && MDIO_DEVRES && MDIO_BUS
 	help
 	  This driver supports NXP ENETC Central MDIO controller as a PCIe
 	  physical function (PF) device.