dt-bindings: input: Add the PinePhone keyboard binding

Add devicetree support for the PinePhone keyboard case, which provides a
matrix keyboard interface and a proxied I2C bus.
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Link: https://lore.kernel.org/r/20220618165747.55709-2-samuel@sholland.orgSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/pine64,pinephone-keyboard.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/input/pine64,pinephone-keyboard.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Pine64 PinePhone keyboard device tree bindings
+
+maintainers:
+  - Samuel Holland <samuel@sholland.org>
+
+description:
+  A keyboard accessory is available for the Pine64 PinePhone and PinePhone Pro.
+  It connects via I2C, providing a raw scan matrix, a flashing interface, and a
+  subordinate I2C bus for communication with a battery charger IC.
+
+properties:
+  compatible:
+    const: pine64,pinephone-keyboard
+
+  reg:
+    const: 0x15
+
+  interrupts:
+    maxItems: 1
+
+  vbat-supply:
+    description: Supply for the keyboard MCU
+
+  wakeup-source: true
+
+  i2c:
+    $ref: /schemas/i2c/i2c-controller.yaml#
+
+required:
+  - compatible
+  - reg
+  - interrupts
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/input/input.h>
+    #include <dt-bindings/interrupt-controller/irq.h>
+
+    i2c {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      keyboard@15 {
+        compatible = "pine64,pinephone-keyboard";
+        reg = <0x15>;
+        interrupt-parent = <&r_pio>;
+        interrupts = <0 12 IRQ_TYPE_EDGE_FALLING>; /* PL12 */
+
+        i2c {
+          #address-cells = <1>;
+          #size-cells = <0>;
+
+          charger@75 {
+            reg = <0x75>;
+          };
+        };
+      };
+    };