dt-bindings: gpio: Add bindings for gpio-virtio

This patch adds binding for virtio GPIO controller, it is based on
virtio-device bindings.
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/acf7402ef4aabc0ad6295c32846f2bef1cd9b56a.1627362340.git.viresh.kumar@linaro.orgSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/gpio/gpio-virtio.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/gpio/gpio-virtio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Virtio GPIO controller
+
+maintainers:
+  - Viresh Kumar <viresh.kumar@linaro.org>
+
+allOf:
+  - $ref: /schemas/virtio/virtio-device.yaml#
+
+description:
+  Virtio GPIO controller, see /schemas/virtio/virtio-device.yaml for more
+  details.
+
+properties:
+  $nodename:
+    const: gpio
+
+  compatible:
+    const: virtio,device29
+
+  gpio-controller: true
+
+  "#gpio-cells":
+    const: 2
+
+  interrupt-controller: true
+
+  "#interrupt-cells":
+    const: 2
+
+required:
+  - compatible
+  - gpio-controller
+  - "#gpio-cells"
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    virtio@3000 {
+        compatible = "virtio,mmio";
+        reg = <0x3000 0x100>;
+        interrupts = <41>;
+
+        gpio {
+            compatible = "virtio,device29";
+            gpio-controller;
+            #gpio-cells = <2>;
+            interrupt-controller;
+            #interrupt-cells = <2>;
+        };
+    };
+
+...