arm64: dts: imx8mp: use syscon for iomuxc-gpr

It is preferred to use syscon per bindind doc
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/imx8mp.dtsi

@@ -414,7 +414,7 @@ iomuxc: pinctrl@30330000 {
 				reg = <0x30330000 0x10000>;
 			};
 
-			gpr: iomuxc-gpr@30340000 {
+			gpr: syscon@30340000 {
 				compatible = "fsl,imx8mp-iomuxc-gpr", "syscon";
 				reg = <0x30340000 0x10000>;
 			};