gpio: pca9570: Add DT bindings for NXP PCA9570

This patch adds device tree bindings for the NXP PCA9570,
a 4-bit I2C GPO expander.
Signed-off-by: Sungbo Eo <mans0n@gorani.run>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200630160934.1197066-1-mans0n@gorani.runSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/gpio-pca9570.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/gpio/gpio-pca9570.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: PCA9570 I2C GPO expander
+
+maintainers:
+  - Sungbo Eo <mans0n@gorani.run>
+
+properties:
+  compatible:
+    enum:
+      - nxp,pca9570
+
+  reg:
+    maxItems: 1
+
+  gpio-controller: true
+
+  '#gpio-cells':
+    const: 2
+
+required:
+  - compatible
+  - reg
+  - gpio-controller
+  - "#gpio-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    i2c0 {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        gpio@24 {
+            compatible = "nxp,pca9570";
+            reg = <0x24>;
+            gpio-controller;
+            #gpio-cells = <2>;
+        };
+    };
+
+...