dt-bindings: iio: stm32-dac: Add support for STM32 DAC

STMicroelectronics STM32 DAC

The STM32 DAC is a 12-bit voltage output digital-to-analog converter. The DAC

may be configured in 8 or 12-bit mode. It has two output channels, each with

its own converter.

It has built-in noise and triangle waveform generator and supports external

triggers for conversions. The DAC's output buffer allows a high drive output

current.

Contents of a stm32 dac root node:

-----------------------------------

Required properties:

- compatible: Must be "st,stm32h7-dac-core".

- reg: Offset and length of the device's register set.

- clocks: Must contain an entry for pclk (which feeds the peripheral bus

  interface)

- clock-names: Must be "pclk".

- vref-supply: Phandle to the vref+ input analog reference supply.

- #address-cells = <1>;

- #size-cells = <0>;

Optional properties:

- resets: Must contain the phandle to the reset controller.

- A pinctrl state named "default" for each DAC channel may be defined to set

  DAC_OUTx pin in mode of operation for analog output on external pin.

Contents of a stm32 dac child node:

-----------------------------------

DAC core node should contain at least one subnode, representing a

DAC instance/channel available on the machine.

Required properties:

- compatible: Must be "st,stm32-dac".

- reg: Must be either 1 or 2, to define (single) channel in use

- #io-channel-cells = <1>: See the IIO bindings section "IIO consumers" in

  Documentation/devicetree/bindings/iio/iio-bindings.txt

Example:

	dac: dac@40007400 {

		compatible = "st,stm32h7-dac-core";

		reg = <0x40007400 0x400>;

		clocks = <&clk>;

		clock-names = "pclk";

		vref-supply = <&reg_vref>;

		pinctrl-names = "default";

		pinctrl-0 = <&dac_out1 &dac_out2>;

		#address-cells = <1>;

		#size-cells = <0>;

		dac1: dac@1 {

			compatible = "st,stm32-dac";

			#io-channels-cells = <1>;

			reg = <1>;

		};

		dac2: dac@2 {

			compatible = "st,stm32-dac";

			#io-channels-cells = <1>;

			reg = <2>;

		};

	};