arm64: dts: imx8mq: Set the correct gpu_ahb clock frequency

According to i.MX 8M Quad Reference Manual, GPU_AHB_CLK_ROOT's maximum
frequency is 400MHz.

Fixes: 45d2c84 ("arm64: dts: imx8mq: add GPU node")
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Sebastian Krzyszkowiak <sebastian.krzyszkowiak@puri.sm>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>

Author: dos1

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

@@ -1632,7 +1632,7 @@
 			                         <&clk IMX8MQ_GPU_PLL_OUT>,
 			                         <&clk IMX8MQ_GPU_PLL>;
 			assigned-clock-rates = <800000000>, <800000000>,
-			                       <800000000>, <800000000>, <0>;
+			                       <800000000>, <400000000>, <0>;
 			power-domains = <&pgc_gpu>;
 		};