net/fec: consolidate all i.MX options to CONFIG_ARM

Moreover stop listing all i.MX platforms featuring a FEC, and use
the platform's config symbol that selects registration of a fec device
instead.  This might make it easier to add new platforms.

Set default = y for ARMs having a fec to reduce defconfig sizes.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>