dt-bindings:iio:adc:adi,ad7476: Add missing binding document

This binding covers class of simple SPI ADCs which only provide
data output - they don't have MOSI pin.

The only real variation between them is over how many supplies they
use and which one is used for the reference.

Michael listed as maintainer for this one as it is his driver and
falls under the catch all MAINTAINERS entry for ADI devices.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210425163154.73209-3-jic23@kernel.org

Documentation/devicetree/bindings/iio/adc/adi,ad7476.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+# Copyright 2019 Analog Devices Inc.
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/adc/adi,ad7476.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: AD7476 and similar simple SPI ADCs from multiple manufacturers.
+
+maintainers:
+  - Michael Hennerich <michael.hennerich@analog.com>
+
+description: |
+  A lot of simple SPI ADCs have very straight forward interfaces.
+  They typically don't provide a MOSI pin, simply reading out data
+  on MISO when the clock toggles.
+
+properties:
+  compatible:
+    enum:
+      - adi,ad7091
+      - adi,ad7091r
+      - adi,ad7273
+      - adi,ad7274
+      - adi,ad7276
+      - adi,ad7277
+      - adi,ad7278
+      - adi,ad7466
+      - adi,ad7467
+      - adi,ad7468
+      - adi,ad7475
+      - adi,ad7476
+      - adi,ad7476a
+      - adi,ad7477
+      - adi,ad7477a
+      - adi,ad7478
+      - adi,ad7478a
+      - adi,ad7495
+      - adi,ad7910
+      - adi,ad7920
+      - adi,ad7940
+      - ti,adc081s
+      - ti,adc101s
+      - ti,adc121s
+      - ti,ads7866
+      - ti,ads7867
+      - ti,ads7868
+      - lltc,ltc2314-14
+
+  reg:
+    maxItems: 1
+
+  vcc-supply:
+    description:
+      Main powersupply voltage for the chips, sometimes referred to as VDD on
+      datasheets.  If there is no separate vref-supply, then this is needed
+      to establish channel scaling.
+
+  vdrive-supply:
+    description:
+      Some devices have separate supply for their digital control side.
+
+  vref-supply:
+    description:
+      Some devices have a specific reference voltage supplied on a different pin
+      to the other supplies. Needed to be able to establish channel scaling
+      unless there is also an internal reference available (e.g. ad7091r)
+
+  spi-max-frequency: true
+
+  adi,conversion-start-gpios:
+    description: A GPIO used to trigger the start of a conversion
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+allOf:
+  # Devices where reference is vcc
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - adi,ad7091
+              - adi,ad7276
+              - adi,ad7277
+              - adi,ad7278
+              - adi,ad7466
+              - adi,ad7467
+              - adi,ad7468
+              - adi,ad7940
+              - ti,adc081s
+              - ti,adc101s
+              - ti,adc121s
+              - ti,ads7866
+              - ti,ads7868
+      required:
+        - vcc-supply
+  # Devices with a vref
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - adi,ad7091r
+              - adi,ad7273
+              - adi,ad7274
+              - adi,ad7475
+              - lltc,ltc2314-14
+    then:
+      properties:
+        vref-supply: true
+    else:
+      properties:
+        vref-supply: false
+  # Devices with a vref where it is not optional
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - adi,ad7273
+              - adi,ad7274
+              - adi,ad7475
+              - lltc,ltc2314-14
+    then:
+      required:
+        - vref-supply
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - adi,ad7475
+              - adi,ad7495
+    then:
+      properties:
+        vdrive-supply: true
+    else:
+      properties:
+        vdrive-supply: false
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - adi,ad7091
+              - adi,ad7091r
+    then:
+      properties:
+        adi,conversion-start-gpios: true
+    else:
+      properties:
+        adi,conversion-start-gpios: false
+
+examples:
+  - |
+    spi {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      adc@0 {
+        compatible = "adi,ad7091r";
+        reg = <0>;
+        spi-max-frequency = <5000000>;
+        vcc-supply = <&adc_vcc>;
+        vref-supply = <&adc_vref>;
+      };
+    };
+...