dt-bindings: pinctrl: rt2880: add binding document

The commit adds rt2880 compatible node in binding document.
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20201208075523.7060-2-sergio.paracuellos@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/pinctrl/ralink,rt2880-pinmux.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/pinctrl/ralink,rt2880-pinmux.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Ralink rt2880 pinmux controller
+
+maintainers:
+  - Sergio Paracuellos <sergio.paracuellos@gmail.com>
+
+description:
+  The rt2880 pinmux can only set the muxing of pin groups. muxing indiviual pins
+  is not supported. There is no pinconf support.
+
+properties:
+  compatible:
+    enum:
+      - ralink,rt2880-pinmux
+
+  pinctrl-0:
+    description:
+      A phandle to the node containing the subnodes containing default
+      configurations. This is for pinctrl hogs.
+
+  pinctrl-names:
+    description:
+      A pinctrl state named "default" can be defined.
+    const: default
+
+required:
+  - compatible
+
+patternProperties:
+  '[a-z0-9_-]+':
+    if:
+      type: object
+      description: node for pinctrl.
+      $ref: "pinmux-node.yaml"
+    then:
+      properties:
+        groups:
+          description: Name of the pin group to use for the functions.
+          enum: [i2c, spi, uart1, uart2, uart3, rgmii1, rgmii2, mdio,
+                 pcie, sdhci]
+        function:
+          description: The mux function to select
+          enum: [gpio, i2c, spi, uart1, uart2, uart3, rgmii1, rgmii2,
+                 mdio, nand1, nand2, sdhci]
+
+additionalProperties: false
+
+examples:
+  # Pinmux controller node
+  - |
+    pinctrl {
+      compatible = "ralink,rt2880-pinmux";
+      pinctrl-names = "default";
+      pinctrl-0 = <&state_default>;
+
+      state_default: pinctrl0 {
+      };
+
+      i2c_pins: i2c0 {
+        i2c0 {
+          groups = "i2c";
+          function = "i2c";
+        };
+      };
+    };