powerpc/corenet: Enable muxing MDIO buses via GPIO

Signed-off-by: Andy Fleming <afleming@gmail.com>
Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Shruti Kanetkar <Kanetkar.Shruti@gmail.com>
Signed-off-by: Emil Medve <Emilian.Medve@Freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>