Fix RGMII-ID handling in gianfar

The TSEC/eTSEC can detect the interface to the PHY automatically,
but it isn't able to detect whether the RGMII connection needs internal
delay.  So we need to detect that change in the device tree, propagate
it to the platform data, and then check it if we're in RGMII.  This fixes
a bug on the 8641D HPCN board where the Vitesse PHY doesn't use the delay
for RGMII.

Signed-off-by: Andy Fleming <afleming@freescale.com>