arm64: dts: imx8mp-verdin: Label ldo5 and link to usdhc2

This commit labels LDO5 as `reg_vdd_sdio` in `imx8mp-verdin.dtsi` to
facilitate changing its voltage to 1.8V, necessary for an SDIO
peripheral that requires 1.8V at default and high-speed modes.

Additionally, it links `reg_vdd_sdio` to `&usdhc2`, aligning with the
hardware configuration specified in the datasheet.

Signed-off-by: Philippe Schenker <philippe.schenker@impulsing.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>