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

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: 45d2c84eb3a2 ("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>