FEC: Turn FEC driver into platform device driver

This turns the fec driver into a platform device driver for new
platforms. Old platforms are still supported through a FEC_LEGACY define
till they are also ported.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>