ARM: dts: imx7ulp: Remove incorrect mmc fallback compatible

ARM: dts: imx7ulp: Remove incorrect mmc fallback compatible

Per fsl-imx-esdhc.yaml, the correct compatible string for i.MX7ULP is:

compatible = "fsl,imx7ulp-usdhc"

Remove the undocumented "fsl,imx6sx-usdhc" fallback compatible.

This fixes the following dt-schema warnings:

mmc@2198000: compatible: 'oneOf' conditional failed, one must be fixed:
	['fsl,imx6sl-usdhc', 'fsl,imx6q-usdhc'] is too long
	['fsl,imx6sl-usdhc', 'fsl,imx6q-usdhc'] is too short
	'fsl,imx50-esdhc' was expected
	...

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>