ARM: dts: stm32: set i2c4 bus freq to 400KHz on stm32mp157c-ed1

On this board, the I2C4 bus has only a single slave (pmic) which
supports I2C Fast Mode hence setting 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/stm32mp157c-ed1.dts

@@ -134,6 +134,7 @@ &i2c4 {
 	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;