ARM: imx: imx6sl iomuxc syscon is compatible to imx6q

The imx6sl iomuxc syscon is compatible to imx6q, so let's add
compatible string 'fsl,imx6q-iomuxc-gpr' for imx6sl iomuxc syscon node.
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>

arch/arm/boot/dts/imx6sl.dtsi

@@ -533,7 +533,8 @@
 			};
 
 			gpr: iomuxc-gpr@020e0000 {
-				compatible = "fsl,imx6sl-iomuxc-gpr", "syscon";
+				compatible = "fsl,imx6sl-iomuxc-gpr",
+					     "fsl,imx6q-iomuxc-gpr", "syscon";
 				reg = <0x020e0000 0x38>;
 			};