ARM: dts: imx: Set default tuning step for imx6sx usdhc

If the tuning step is not set, the tuning step is set to 1.
For some sd cards, the following Tuning timeout will occur.

Tuning failed, falling back to fixed sampling clock

So set the default tuning step. This refers to the NXP vendor's
commit below:

https://github.com/nxp-imx/linux-imx/blob/lf-6.1.y/


arch/arm/boot/dts/imx6sx.dtsi#L1108-L1109

Fixes: 1e336aa0 ("mmc: sdhci-esdhc-imx: correct the tuning start tap and step setting")
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>