arm64: dts: imx8mn: Slow default video_pll clock rate

Since commit 8208181f ("clk: imx: composite-8m:
Add imx8m_divider_determine_rate") the lcdif controller has
had the ability to set the disp_pixel_clk rate which propagates
up the tree and sets the video_pll rate automatically.

By setting this value low, it will force the recalculation of
video_pll to the lowest rate needed by lcdif instead of
dividing a larger clock down to the desired clock speed. This
has the  advantage of being able to lower the video_pll rate
from 594MHz to 148.5MHz when operating at 1080p. It can go even
lower when operating at lower resolutions and refresh rates.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>