drivers: net: add Layerscape mEMAC MDIO driver

Add a driver for the MDIO interface integrated in the mEMAC (Multi-rate
Ethernet Media Access Controller) and the Fman 10G Ethernet MACs.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>