ARM: imx6sl: config iomux-gpr1 to select clock for fec

Config iomux-gpr1 to select clock source for fec system clock.
Clear gpr1[14], gpr1[18-17] bit to select the fec clock source
from internal anatop PLL.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>