dt-bindings: input: Add docs for ADC driven joystick

Add documentation for the adc-joystick driver, used to provide support
for joysticks connected over ADC.
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Tested-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200927123302.31062-1-contact@artur-rojek.euSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/adc-joystick.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+# Copyright 2019-2020 Artur Rojek
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/input/adc-joystick.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: ADC attached joystick
+
+maintainers:
+  - Artur Rojek <contact@artur-rojek.eu>
+
+description: >
+  Bindings for joystick devices connected to ADC controllers supporting
+  the Industrial I/O subsystem.
+
+properties:
+  compatible:
+    const: adc-joystick
+
+  io-channels:
+    minItems: 1
+    maxItems: 1024
+    description: >
+      List of phandle and IIO specifier pairs.
+      Each pair defines one ADC channel to which a joystick axis is connected.
+      See Documentation/devicetree/bindings/iio/iio-bindings.txt for details.
+
+  '#address-cells':
+    const: 1
+
+  '#size-cells':
+    const: 0
+
+required:
+  - compatible
+  - io-channels
+  - '#address-cells'
+  - '#size-cells'
+
+additionalProperties: false
+
+patternProperties:
+  "^axis@[0-9a-f]+$":
+    type: object
+    description: >
+      Represents a joystick axis bound to the given ADC channel.
+      For each entry in the io-channels list, one axis subnode with a matching
+      reg property must be specified.
+
+    properties:
+      reg:
+        minimum: 0
+        maximum: 1023
+        description: Index of an io-channels list entry bound to this axis.
+
+      linux,code:
+        $ref: /schemas/types.yaml#/definitions/uint32
+        description: EV_ABS specific event code generated by the axis.
+
+      abs-range:
+        allOf:
+          - $ref: /schemas/types.yaml#/definitions/uint32-array
+          - items:
+              - description: minimum value
+              - description: maximum value
+        description: >
+          Minimum and maximum values produced by the axis.
+          For an ABS_X axis this will be the left-most and right-most
+          inclination of the joystick. If min > max, it is left to userspace to
+          treat the axis as inverted.
+          This property is interpreted as two signed 32 bit values.
+
+      abs-fuzz:
+        $ref: /schemas/types.yaml#/definitions/uint32
+        description: >
+          Amount of noise in the input value.
+          Omitting this property indicates the axis is precise.
+
+      abs-flat:
+        $ref: /schemas/types.yaml#/definitions/uint32
+        description: >
+          Axial "deadzone", or area around the center position, where the axis
+          is considered to be at rest.
+          Omitting this property indicates the axis always returns to exactly
+          the center position.
+
+    required:
+      - reg
+      - linux,code
+      - abs-range
+
+    additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/iio/adc/ingenic,adc.h>
+    #include <dt-bindings/input/input.h>
+
+    joystick: adc-joystick {
+      compatible = "adc-joystick";
+      io-channels = <&adc INGENIC_ADC_TOUCH_XP>,
+                    <&adc INGENIC_ADC_TOUCH_YP>;
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      axis@0 {
+              reg = <0>;
+              linux,code = <ABS_X>;
+              abs-range = <3300 0>;
+              abs-fuzz = <4>;
+              abs-flat = <200>;
+      };
+      axis@1 {
+              reg = <1>;
+              linux,code = <ABS_Y>;
+              abs-range = <0 3300>;
+              abs-fuzz = <4>;
+              abs-flat = <200>;
+      };
+    };