drivers: net: xgene: Enable MDIO driver

This patch enables MDIO driver by,

- Selecting MDIO_XGENE
- Changed open and close to use phy_start and phy_stop
- Changed to use mac_ops->tx(rx)_enable and tx(rx)_disable

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Tested-by: Fushen Chen <fchen@apm.com>
Tested-by: Toan Le <toanle@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>