dt-bindings: gpio: Add imx scu gpio driver bindings

Add binding document for the imx scu gpio driver.
Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>

Documentation/devicetree/bindings/gpio/fsl,imx8qxp-sc-gpio.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/gpio/fsl,imx8qxp-sc-gpio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: GPIO driver over IMX SCU firmware API
+
+maintainers:
+  - Shenwei Wang <shenwei.wang@nxp.com>
+
+description: |
+  This module provides the standard interface to control the
+  resource pins in SCU domain on i.MX8 platforms.
+
+properties:
+  compatible:
+    enum:
+      - fsl,imx8qxp-sc-gpio
+
+  "#gpio-cells":
+    const: 2
+
+  gpio-controller: true
+
+required:
+  - compatible
+  - "#gpio-cells"
+  - gpio-controller
+
+additionalProperties: false
+
+examples:
+  - |
+    gpio0: gpio {
+        compatible = "fsl,imx8qxp-sc-gpio";
+        gpio-controller;
+        #gpio-cells = <2>;
+    };