regulator: Add STM32 Voltage Reference Buffer

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>;

	};