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>