arm64: dts: imx8mp-phycore-som: Adapt eMMC drive strength

Set eMMC drive strength for USDHC3_DATA lines (200Mhz)
to X4 for signal improvement.

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>