arm64: dts: imx8mm: correct iomuxc-gpr compatible

The IOMUX Controller General purpose register group are unique almost
per SoC, i.MX8MM is not compatible with i.MX6Q. So correct it.
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mm.dtsi

@@ -552,8 +552,8 @@ iomuxc: pinctrl@30330000 {
 				reg = <0x30330000 0x10000>;
 			};
 
-			gpr: iomuxc-gpr@30340000 {
-				compatible = "fsl,imx8mm-iomuxc-gpr", "fsl,imx6q-iomuxc-gpr", "syscon";
+			gpr: syscon@30340000 {
+				compatible = "fsl,imx8mm-iomuxc-gpr", "syscon";
 				reg = <0x30340000 0x10000>;
 			};