ARM: mx50: Select IMX_HAVE_PLATFORM_FEC as done on other i.MX devices

Currently it is only the MX50_RDP board that selects IMX_HAVE_PLATFORM_FEC in its board Kconfig.

Do like the other i.MX processors, which select IMX_HAVE_PLATFORM_FEC based on the processor type.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>