ARM: dts: imx7d-pico: 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: f13f571a ("ARM: dts: imx7d-pico: Extend peripherals support")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx7d-pico.dtsi

@@ -351,7 +351,7 @@ &usdhc1 {
 	pinctrl-2 = <&pinctrl_usdhc1_200mhz>;
 	cd-gpios = <&gpio5 0 GPIO_ACTIVE_LOW>;
 	bus-width = <4>;
-	tuning-step = <2>;
+	fsl,tuning-step = <2>;
 	vmmc-supply = <&reg_3p3v>;
 	wakeup-source;
 	no-1-8-v;