net/fsl: remove dependency FSL_SOC from MDIO

FSL_PQ_MDIO and FSL_XGMAC_MDIO are not really depend on FSL_SOC, they
can build on non-PPC platforms.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>