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

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: ef484dfcf6f7 ("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>