dt-bindings: net: add amlogic gxl mdio multiplexer

Add documentation for the MDIO bus multiplexer found on the Amlogic GXL
SoC family
Reviewed-by: NKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: NJerome Brunet <jbrunet@baylibre.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/amlogic,gxl-mdio-mux.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/net/amlogic,gxl-mdio-mux.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Amlogic GXL MDIO bus multiplexer
+
+maintainers:
+  - Jerome Brunet <jbrunet@baylibre.com>
+
+description:
+  This is a special case of a MDIO bus multiplexer. It allows to choose between
+  the internal mdio bus leading to the embedded 10/100 PHY or the external
+  MDIO bus on the Amlogic GXL SoC family.
+
+allOf:
+  - $ref: mdio-mux.yaml#
+
+properties:
+  compatible:
+    const: amlogic,gxl-mdio-mux
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    items:
+      - const: ref
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - clock-names
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    eth_phy_mux: mdio@558 {
+      compatible = "amlogic,gxl-mdio-mux";
+      reg = <0x558 0xc>;
+      #address-cells = <1>;
+      #size-cells = <0>;
+      clocks = <&refclk>;
+      clock-names = "ref";
+      mdio-parent-bus = <&mdio0>;
+
+      external_mdio: mdio@0 {
+        reg = <0x0>;
+        #address-cells = <1>;
+        #size-cells = <0>;
+      };
+
+      internal_mdio: mdio@1 {
+        reg = <0x1>;
+        #address-cells = <1>;
+        #size-cells = <0>;
+      };
+    };