ARM: dts: imx25-pdk: Add FEC pins

Instead of relying on the bootloader for configuring the FEC pins, pass the
FEC pin configuration via device tree instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>