arm64: dts: imx8mm-venice-gw7901: Increase HS400 USDHC clock speed

arm64: dts: imx8mm-venice-gw7901: 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: 2b1649a83afc ("arm64: dts: imx: Add i.mx8mm Gateworks gw7901 dts support")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Link: https://lore.kernel.org/stable/20250707201702.2930066-3-tharvey%40gateworks.com
Signed-off-by: Shawn Guo <shawnguo@kernel.org>