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>

arch/arm/boot/dts/imx6sx.dtsi

@@ -697,7 +697,8 @@ iomuxc: iomuxc@020e0000 {
 			};
 
 			gpr: iomuxc-gpr@020e4000 {
-				compatible = "fsl,imx6sx-iomuxc-gpr", "syscon";
+				compatible = "fsl,imx6sx-iomuxc-gpr",
+					     "fsl,imx6q-iomuxc-gpr", "syscon";
 				reg = <0x020e4000 0x4000>;
 			};