dt-bindings: mmc: fsl-imx-esdhc: Add imx6ul support

Add the 'fsl,imx6ul-usdhc' value to the compatible properties list in
the fsl-imx-esdhc.yaml file. This is required to match the compatible
strings present in the 'mmc@2190000' node of 'imx6ul-prti6g.dtb'. This
commit addresses the following dtbs_check warning:
imx6ul-prti6g.dtb:0:0: /soc/bus@2100000/mmc@2190000: failed to match any schema with compatible: ['fsl,imx6ul-usdhc', 'fsl,imx6sx-usdhc']

Signed-off-by: Oleksij Rempel <[e-mail]>
Acked-by: Conor Dooley <[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

···
           - enum:
               - fsl,imx6sll-usdhc
               - fsl,imx6ull-usdhc
+              - fsl,imx6ul-usdhc
           - const: fsl,imx6sx-usdhc
       - items:
           - const: fsl,imx7d-usdhc