ARM: dts: stm32: Enable VREFBUF on stm32mp157a-dk1

Enable VREFBUF as ADC/DAC uses it on stm32mp157a-dk1 board.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp157a-dk1.dts

@@ -449,3 +449,10 @@ &uart4 {
 	pinctrl-0 = <&uart4_pins_a>;
 	status = "okay";
 };
+
+&vrefbuf {
+	regulator-min-microvolt = <2500000>;
+	regulator-max-microvolt = <2500000>;
+	vdda-supply = <&vdd>;
+	status = "okay";
+};