ARM: imx6sx: add imx6sx iomux-gpr field define

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>
(cherry picked from commit 49c71d1c3dccff640cc082875bd8d62988d63df9)
Signed-off-by: Alex Shi <alex.shi@linaro.org>