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>