regulator: Add STM32 Voltage Reference Buffer

Document STM32 VREFBUF (voltage reference buffer) which can be used as
voltage reference for ADCs, DACs and external components.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/st,stm32-vrefbuf.txt

+STM32 VREFBUF - Voltage reference buffer
+
+Some STM32 devices embed a voltage reference buffer which can be used as
+voltage reference for ADCs, DACs and also as voltage reference for external
+components through the dedicated VREF+ pin.
+
+Required properties:
+- compatible:		Must be "st,stm32-vrefbuf".
+- reg:			Offset and length of VREFBUF register set.
+- clocks:		Must contain an entry for peripheral clock.
+
+Example:
+	vrefbuf: regulator@58003C00 {
+		compatible = "st,stm32-vrefbuf";
+		reg = <0x58003C00 0x8>;
+		clocks = <&rcc VREF_CK>;
+		regulator-min-microvolt = <1500000>;
+		regulator-max-microvolt = <2500000>;
+		vdda-supply = <&vdda>;
+	};