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

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

The compatible string "fsl,imx8qm-usdhc" was wrongly dropped in commit
80fd350b9590 ("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: 80fd350b9590 ("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>