dt-bindings: mmc: fsl-imx-esdhc: add imx6ull compatible string

Add a imx6ull compatible string to be able to manage erratum ERR010450 on
i.MX6ULL.
Signed-off-by: NHaibo Chen <haibo.chen@nxp.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

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

@@ -15,6 +15,7 @@ Required properties:
 	       "fsl,imx6q-usdhc"
 	       "fsl,imx6sl-usdhc"
 	       "fsl,imx6sx-usdhc"
+	       "fsl,imx6ull-usdhc"
 	       "fsl,imx7d-usdhc"
 	       "fsl,imx8qxp-usdhc"