arm64: dts: imx8mp-phycore-som: Set usdhc root clock for eMMC

Set the usdhc root clock to 400MHz to be able to support
HS400/HS400ES modes for eMMC on phyCORE-i.MX8MP SoM.

Signed-off-by: Jonas Kuenstler <j.kuenstler@phytec.de>
Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Haibo Chen <haibo.chen@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>