Add imx6sx iomux-gpr register field define in "imx6q-iomuxc-gpr.h" header
file, which is not fully define all iomux-gpr registers and fields, only
align with freescale internal tree related GPR macro define.
Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>