dt-bindings: gpio: Add Loongson-1 GPIO

Add devicetree binding document for Loongson-1 GPIO.
Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Documentation/devicetree/bindings/gpio/loongson,ls1x-gpio.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/gpio/loongson,ls1x-gpio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Loongson-1 GPIO controller
+
+maintainers:
+  - Keguang Zhang <keguang.zhang@gmail.com>
+
+properties:
+  compatible:
+    const: loongson,ls1x-gpio
+
+  reg:
+    maxItems: 1
+
+  gpio-controller: true
+
+  "#gpio-cells":
+    const: 2
+
+  ngpios:
+    minimum: 1
+    maximum: 32
+
+required:
+  - compatible
+  - reg
+  - gpio-controller
+  - "#gpio-cells"
+  - ngpios
+
+additionalProperties: false
+
+examples:
+  - |
+    gpio0: gpio@1fd010c0 {
+        compatible = "loongson,ls1x-gpio";
+        reg = <0x1fd010c0 0x4>;
+
+        gpio-controller;
+        #gpio-cells = <2>;
+
+        ngpios = <32>;
+    };
+
+...