dts: dac: add a new binding for the SAM0 DAC

# Copyright (c) 2020 Google LLC.

# SPDX-License-Identifier: Apache-2.0

description: Atmel SAM0 family DAC

compatible: "atmel,sam0-dac"

include: dac-controller.yaml

properties:

    reg:

      required: true

    clocks:

      required: true

    clock-names:

      required: true

    label:

      required: true

    reference:

      type: string

      required: false

      description: Reference voltage source

      enum:

        - "intref"

        - "vddana"

        - "vrefa"

    "#io-channel-cells":

      const: 0