ARM: imx6sl: add imx6sl iomux-gpr field define

Add imx6sl iomux-gpr register field define in "imx6q-iomuxc-gpr.h" header
file, which is not fully define all iomux-gpr registers and fields, only
add fec related macro define.

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