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>