arm64: dts: imx8mm: Slow default video_pll1 clock rate

Since commit 8208181f ("clk: imx: composite-8m:
Add imx8m_divider_determine_rate") the lcdif controller has
had the ability to set the lcdif_pixel rate which propagates
up the tree and sets the video_pll1 rate automatically.

By setting this value low, it will force the recalculation of
video_pll1 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_pll1 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>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de> # Kontron BL i.MX8MM
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mm.dtsi

@@ -1133,7 +1133,7 @@ lcdif: lcdif@32e00000 {
 				assigned-clock-parents = <&clk IMX8MM_VIDEO_PLL1_OUT>,
 							 <&clk IMX8MM_SYS_PLL2_1000M>,
 							 <&clk IMX8MM_SYS_PLL1_800M>;
-				assigned-clock-rates = <594000000>, <500000000>, <200000000>;
+				assigned-clock-rates = <24000000>, <500000000>, <200000000>;
 				interrupts = <GIC_SPI 5 IRQ_TYPE_LEVEL_HIGH>;
 				power-domains = <&disp_blk_ctrl IMX8MM_DISPBLK_PD_LCDIF>;
 				status = "disabled";