drivers: net: xgene: Use rgmii mdio mac access

This patch switches to use rgmii mdio mac access routines if available,
as they share the same HW.

Signed-off-by: Quan Nguyen <qnguyen@apm.com>
Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>