dt-bindings: net: Add bcm6368-mdio-mux bindings

Add documentations for bcm6368 mdio mux driver.
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/brcm,bcm6368-mdio-mux.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/net/brcm,bcm6368-mdio-mux.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Broadcom BCM6368 MDIO bus multiplexer
+
+maintainers:
+  - Álvaro Fernández Rojas <noltari@gmail.com>
+
+description:
+  This MDIO bus multiplexer defines buses that could be internal as well as
+  external to SoCs. When child bus is selected, one needs to select these two
+  properties as well to generate desired MDIO transaction on appropriate bus.
+
+allOf:
+  - $ref: "mdio.yaml#"
+
+properties:
+  compatible:
+    const: brcm,bcm6368-mdio-mux
+
+  "#address-cells":
+    const: 1
+
+  "#size-cells":
+    const: 0
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+patternProperties:
+  '^mdio@[0-1]$':
+    type: object
+    properties:
+      reg:
+        maxItems: 1
+
+      "#address-cells":
+        const: 1
+
+      "#size-cells":
+        const: 0
+
+    required:
+      - reg
+      - "#address-cells"
+      - "#size-cells"
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    mdio0: mdio@10e000b0 {
+      #address-cells = <1>;
+      #size-cells = <0>;
+      compatible = "brcm,bcm6368-mdio-mux";
+      reg = <0x10e000b0 0x6>;
+
+      mdio_int: mdio@0 {
+        #address-cells = <1>;
+        #size-cells = <0>;
+        reg = <0>;
+      };
+
+      mdio_ext: mdio@1 {
+        #address-cells = <1>;
+        #size-cells = <0>;
+        reg = <1>;
+      };
+    };