dt-bindings: mmc: fsl-imx-esdhc: add a new compatible string

Lack a compatible string "fsl,imx6sll-usdhc", so add it here.

Signed-off-by: Haibo Chen <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

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

···
           - fsl,imx53-esdhc
           - fsl,imx6q-usdhc
           - fsl,imx6sl-usdhc
+          - fsl,imx6sll-usdhc
           - fsl,imx6sx-usdhc
           - fsl,imx6ull-usdhc
           - fsl,imx7d-usdhc