'power-supply' is an invalid property for usdhc and it gives
the following schema warning:

imx8mq-librem5-r2.dtb: mmc@30b40000: Unevaluated properties are not allowed ('power-supply' was unexpected)
From schema: Documentation/devicetree/bindings/mmc/fsl-imx-esdhc.yaml

Change it to 'vqmmc-supply', which represents the supply for the bus IO.
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>