fec: add support for PHY interface platform data

The i.MX25 PDK uses RMII to communicate with its PHY. This patch adds
the ability to configure RMII, based on platform data.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>