ARM: dts: imx6ul-pico-dwarf: Use 'clock-frequency'

'clock_frequency' is not a valid property.

Use the correct 'clock-frequency' instead.

Fixes: 47246faf ("ARM: dts: imx6ul-pico: Add support for the dwarf baseboard")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6ul-pico-dwarf.dts

@@ -32,7 +32,7 @@ sys_mclk: clock-sys-mclk {
 };
 
 &i2c2 {
-	clock_frequency = <100000>;
+	clock-frequency = <100000>;
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_i2c2>;
 	status = "okay";