arm64: dts: imx8mq: correct iomuxc-gpr compatible

The IOMUX Controller General purpose register group are unique almost
per SoC, i.MX8MQ is not compatible with i.MX6Q. So correct it.
Also update name with syscon.
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mq.dtsi

@@ -575,8 +575,7 @@ iomuxc: pinctrl@30330000 {
 			};
 
 			iomuxc_gpr: syscon@30340000 {
-				compatible = "fsl,imx8mq-iomuxc-gpr", "fsl,imx6q-iomuxc-gpr",
-					     "syscon", "simple-mfd";
+				compatible = "fsl,imx8mq-iomuxc-gpr", "syscon", "simple-mfd";
 				reg = <0x30340000 0x10000>;
 
 				mux: mux-controller {