ARM: dts: imx6sx: iomux-gpr syscon is compatible to imx6q

The imx6sx iomuxc-gpr syscon is compatible to imx6q, so let's add
compatible string 'fsl,imx6q-iomuxc-gpr' for imx6sx iomuxc-gpr syscon node.

This is necessary to enable SW workaround for ERR007265,
please refer to imx6_pm_common_init of arch/arm/mach-imx/pm-imx6.c
for detail.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>