The i.MX 8 DTSes use two compatibles so update the binding to fix
dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mn-evk.dt.yaml: mmc@30b40000:
    compatible: ['fsl,imx8mn-usdhc', 'fsl,imx7d-usdhc'] is too long
    From schema: Documentation/devicetree/bindings/mmc/fsl-imx-esdhc.yaml

  arch/arm64/boot/dts/freescale/imx8mn-evk.dt.yaml: mmc@30b40000:
    compatible: Additional items are not allowed ('fsl,imx7d-usdhc' was unexpected)

  arch/arm64/boot/dts/freescale/imx8mn-ddr4-evk.dt.yaml: mmc@30b40000:
    compatible: ['fsl,imx8mn-usdhc', 'fsl,imx7d-usdhc'] is too long
Signed-off-by: NKrzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200829062505.4642-1-krzk@kernel.orgSigned-off-by: NUlf Hansson <ulf.hansson@linaro.org>