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 <o.rempel@pengutronix.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230621093245.78130-2-o.rempel@pengutronix.deSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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

@@ -42,6 +42,7 @@ properties:
           - enum:
               - fsl,imx6sll-usdhc
               - fsl,imx6ull-usdhc
+              - fsl,imx6ul-usdhc
           - const: fsl,imx6sx-usdhc
       - items:
           - const: fsl,imx7d-usdhc