Change UEC PHY interface to RGMII on MPC8568MDS

Change UEC phy interface from GMII to RGMII on MPC8568MDS board

Because on MPC8568MDS, GMII interface is only recommended for 1000Mbps speed,
but RGMII interface can work at 10/100/1000Mbps, and RGMII interface works more stable.

Now both UEC1 and UEC2 can work properly under u-boot.

It is also in consistent with the kernel setting for 8568 UEC phy interface.

Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>