mmc: sdhci-esdhc-imx: using specific compatible string in binding doc

Using specific compatible string in binding doc to make the binding
more clear.
It's also used to avoid checkpatch warning in the future like follows:
WARNING: DT compatible string "fsl,imx6sx-usdhc" appears un-documented --
check ./Documentation/devicetree/bindings/
+       { .compatible = "fsl,imx6sx-usdhc", .data = &usdhc_imx6sx_data, },

total: 0 errors, 1 warnings, 18 lines checked
Signed-off-by: Dong Aisheng <b29396@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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

@@ -7,7 +7,14 @@ This file documents differences between the core properties described
 by mmc.txt and the properties used by the sdhci-esdhc-imx driver.
 
 Required properties:
-- compatible : Should be "fsl,<chip>-esdhc"
+- compatible : Should be "fsl,<chip>-esdhc", the supported chips include
+	       "fsl,imx25-esdhc"
+	       "fsl,imx35-esdhc"
+	       "fsl,imx51-esdhc"
+	       "fsl,imx53-esdhc"
+	       "fsl,imx6q-usdhc"
+	       "fsl,imx6sl-usdhc"
+	       "fsl,imx6sx-usdhc"
 
 Optional properties:
 - fsl,cd-controller : Indicate to use controller internal card detection