udoo_neo: Fix ethernet

The ethernet has a RMII not RGMII, also needs DM_MDIO and finally
initialise it later in the process as it's not needed that early on
and not everything is ready so it locks up the device.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>