ARM: dts: imx7d-meerkat96: Fix the 'tuning-step' property

According to Documentation/devicetree/bindings/mmc/fsl-imx-esdhc.yaml, the
correct name of the property is 'fsl,tuning-step'.

Fix it accordingly.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Fixes: ae7b3384

 ("ARM: dts: Add support for 96Boards Meerkat96 board")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>