mmc: sdhci-esdhc-imx: add tuning-step setting support

tuning-step is the delay cell steps in tuning procedure. The default value
of tuning-step is 1. Some boards or cards need another value to pass the
tuning procedure. For example, imx7d-sdb board need the tuning-step value
as 2, otherwise it can't pass the tuning procedure.

So this patch add the tuning-step setting in driver, so that user can set
the tuning-step value in dts.

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>