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>

arch/arm/boot/dts/imx7d-meerkat96.dts

@@ -193,7 +193,7 @@ &usdhc1 {
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_usdhc1>;
 	keep-power-in-suspend;
-	tuning-step = <2>;
+	fsl,tuning-step = <2>;
 	vmmc-supply = <&reg_3p3v>;
 	no-1-8-v;
 	broken-cd;