Fix Vitesse RGMII-ID support

The Vitesse PHY on the 8641D needs to be set up with internal delay to
work in RGMII mode.  So we add skew when it is set to RGMII_ID mode.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Haruki Dai <Dai.Haruki@freescale.com>
Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>