Add a required property "fsl,imx7d-usdhc" in binding doc.
Add an optional property "fsl,tuning-step" in binding doc.
Signed-off-by: Haibo Chen <haibo.chen@freescale.com>
Acked-by: Dong Aisheng <aisheng.dong@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>