net/fsl: Add mEMAC MDIO support to XGMAC MDIO

The Freescale mEMAC supports operating at 10/100/1000/10G, and
its associated MDIO controller is likewise capable of operating
both Clause 22 and Clause 45 MDIO buses. It is nearly identical
to the MDIO controller on the XGMAC, so we just modify that
driver.

Portions of this driver developed by:

Sandeep Singh <sandeep@freescale.com>
Roy Zang <tie-fei.zang@freescale.com>

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