ARM: dts: stm32: set i2c4 bus freq to 400KHz on stm32mp15 DK boards

On DK boards, all I2C4 bus slaves supports I2C Fast Mode hence setting
the bus frequency to 400 KHz.

Signed-off-by: Alain Volmat <alain.volmat@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp15xx-dkx.dtsi

@@ -220,6 +220,7 @@
 	pinctrl-0 = <&i2c4_pins_a>;
 	i2c-scl-rising-time-ns = <185>;
 	i2c-scl-falling-time-ns = <20>;
+	clock-frequency = <400000>;
 	status = "okay";
 	/* spare dmas for other usage */
 	/delete-property/dmas;