dt-bindings: input: Add Dell Wyse 3020 Power Button binding

Add binding document for the Dell Wyse 3020 a.k.a. "Ariel" Power Button.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201201083533.1724287-2-lkundrak@v3.skSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/ariel-pwrbutton.yaml

+# SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/input/ariel-pwrbutton.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Dell Wyse 3020 a.k.a. "Ariel" Power Button
+
+maintainers:
+  - Lubomir Rintel <lkundrak@v3.sk>
+
+description: |
+  The ENE Embedded Controller on the Ariel board has an interface to the
+  SPI bus that is capable of sending keyboard and mouse data. A single
+  power button is attached to it. This binding describes this
+  configuration.
+
+allOf:
+  - $ref: input.yaml#
+
+properties:
+  compatible:
+    items:
+      - const: dell,wyse-ariel-ec-input
+      - const: ene,kb3930-input
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  spi-max-frequency: true
+
+required:
+  - compatible
+  - reg
+  - interrupts
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/irq.h>
+
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        power-button@0 {
+            compatible = "dell,wyse-ariel-ec-input", "ene,kb3930-input";
+            reg = <0>;
+            interrupt-parent = <&gpio>;
+            interrupts = <60 IRQ_TYPE_EDGE_RISING>;
+            spi-max-frequency = <33000000>;
+        };
+    };