dt-bindings: add Qualcomm IPQ4019 MDIO bindings

This patch adds the binding document for the IPQ40xx MDIO driver.
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/qcom,ipq4019-mdio.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/net/qcom,ipq4019-mdio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Qualcomm IPQ40xx MDIO Controller Device Tree Bindings
+
+maintainers:
+  - Robert Marko <robert.marko@sartura.hr>
+
+allOf:
+  - $ref: "mdio.yaml#"
+
+properties:
+  compatible:
+    const: qcom,ipq4019-mdio
+
+  "#address-cells":
+    const: 1
+
+  "#size-cells":
+    const: 0
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - "#address-cells"
+  - "#size-cells"
+
+examples:
+  - |
+    mdio@90000 {
+      #address-cells = <1>;
+      #size-cells = <0>;
+      compatible = "qcom,ipq4019-mdio";
+      reg = <0x90000 0x64>;
+
+      ethphy0: ethernet-phy@0 {
+        reg = <0>;
+      };
+
+      ethphy1: ethernet-phy@1 {
+        reg = <1>;
+      };
+
+      ethphy2: ethernet-phy@2 {
+        reg = <2>;
+      };
+
+      ethphy3: ethernet-phy@3 {
+        reg = <3>;
+      };
+
+      ethphy4: ethernet-phy@4 {
+        reg = <4>;
+      };
+    };