arm64: dts: imx8mn-venice-gw7902: Increase HS400 USDHC clock speed

The IMX8M reference manuals indicate in the USDHC Clock generator section
that the clock rate for DDR is 1/2 the input clock therefore HS400 rates
clocked at 200Mhz require a 400Mhz SDHC clock.

This showed about a 1.5x improvement in read performance for the eMMC's
used on the various imx8m{m,n,p}-venice boards.

Fixes: ef484df ("arm64: dts: imx: Add i.mx8mm/imx8mn Gateworks gw7902 dts support")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Author: Gateworks

arch/arm64/boot/dts/freescale/imx8mn-venice-gw7902.dts

@@ -693,6 +693,8 @@
 	pinctrl-0 = <&pinctrl_usdhc3>;
 	pinctrl-1 = <&pinctrl_usdhc3_100mhz>;
 	pinctrl-2 = <&pinctrl_usdhc3_200mhz>;
+	assigned-clocks = <&clk IMX8MN_CLK_USDHC3>;
+	assigned-clock-rates = <400000000>;
 	bus-width = <8>;
 	non-removable;
 	status = "okay";