arm64: dts: imx8mp: Set SPI NOR to max 40 MHz on Data Modul i.MX8M Plus eDM SBC

The SPI NOR bus routing on this board cannot go above 50 MHz,
set the clock frequency to maximum of 40 MHz to be within a
safe margin. Remove the comment as well.

Fixes: 562d222f ("arm64: dts: imx8mp: Add support for Data Modul i.MX8M Plus eDM SBC")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mp-data-modul-edm-sbc.dts

@@ -153,7 +153,7 @@ &ecspi1 {
 	flash@0 {	/* W25Q128JVEI */
 		compatible = "jedec,spi-nor";
 		reg = <0>;
-		spi-max-frequency = <100000000>;	/* Up to 133 MHz */
+		spi-max-frequency = <40000000>;
 		spi-tx-bus-width = <1>;
 		spi-rx-bus-width = <1>;
 	};