Also use hwconfig to configure whether the board has a FEC or not.
We then can adjust the DTS to tell Linux if there is a FEC present.

syntax:

hwconfig=fec:on   if hardware has a  FEC
hwconfig=fec:off  if hardware has no FEC

Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Wolfgang Denk <wd@denx.de>