MLK-25454-1 dts: iMX8MP_DDR4: Set LCDIF/HDMI AXI clock rate to nominal mode

MLK-25454-1 dts: iMX8MP_DDR4: Set LCDIF/HDMI AXI clock rate to nominal mode

According IMX8MPIEC, both LCIDF and HDMI AXI clock rate should set
to nominal mode for iMX8MP DDR4 board.
Clock root         Nominal mode Overdrive mode   Unit
MEDIA_AXI_CLK_ROOT    400           500           MHz
HDMI_AXI_CLK_ROOT     400           500           MHz

Signed-off-by: Sandor Yu <Sandor.yu@nxp.com>
Reviewed-by: Jacky Bai <ping.bai@nxp.com>
(cherry picked from commit 4e698891f8d8c6bf6a3f6bf870bef1bb603dcddd)