dt-bindings: mmc: imx: fix the wrongly dropped imx8qm compatible string

The compatible string "fsl,imx8qm-usdhc" was wrongly dropped in commit
80fd350b95 ("dt-bindings: mmc: fsl-imx-esdhc: Fix i.MX 8 compatible
matching"), let's add it back.

Cc: Haibo Chen <haibo.chen@nxp.com>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Fixes: 80fd350b95 ("dt-bindings: mmc: fsl-imx-esdhc: Fix i.MX 8 compatible matching")
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201126105900.26658-2-aisheng.dong@nxp.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/fsl-imx-esdhc.yaml

@@ -39,6 +39,7 @@ properties:
               - fsl,imx8mn-usdhc
               - fsl,imx8mp-usdhc
               - fsl,imx8mq-usdhc
+              - fsl,imx8qm-usdhc
               - fsl,imx8qxp-usdhc
           - const: fsl,imx7d-usdhc