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>

diff --git a/Documentation/devicetree/bindings/regulator/st,stm32-vrefbuf.txt b/Documentation/devicetree/bindings/regulator/st,stm32-vrefbuf.txt
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index 0000000..3944ee3
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+++ b/Documentation/devicetree/bindings/regulator/st,stm32-vrefbuf.txt
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+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>;
+       };