arm64: dts: imx8mm-venice-gw700x: Increase HS400 USDHC clock speed

arm64: dts: imx8mm-venice-gw700x: 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: 6f30b27c5ef5 ("arm64: dts: imx8mm: Add Gateworks i.MX 8M Mini Development Kits")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>