net: fec: Select the FEC driver by default for i.MX SoCs

Since commit 230dec61

 (net/fec: add imx6q enet support) the FEC driver is no 
longer built by default for i.MX SoCs.

Let the FEC driver be built by default again.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>