LF-1790 arm64: dts: imx8mn-ddr4-evk: Correct GPU clock rate

LF-1790 arm64: dts: imx8mn-ddr4-evk: Correct GPU clock rate

For i.MX8MN DDR4 EVK board, system runs at nominal mode, so GPU can
ONLY run up to 400MHz, correct the clock settings, new GPU clock
settings on i.MX8MN DDR4 EVK board are as below:

root@imx8mnevk:~# cat /sys/kernel/debug/clk/clk_summary | grep gpu
          gpu_ahb                     0        0        0   400000000          0     0  50000
          gpu_axi                     0        0        0   800000000          0     0  50000
             gpu_root_clk             0        0        0   800000000          0     0  50000
    gpu_pll_ref_sel                   0        0        0    24000000          0     0  50000
       gpu_pll                        0        0        0  1200000000          0     0  50000
          gpu_pll_bypass              0        0        0  1200000000          0     0  50000
             gpu_pll_out              0        0        0  1200000000          0     0  50000
                gpu_shader            0        0        0   400000000          0     0  50000
                gpu_core              0        0        0   400000000          0     0  50000
                   gpu_core_root_clk       0        0        0   400000000          0     0  50000

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Jacky Bai <ping.bai@nxp.com>