dt-bindings: net: Add aspeed, ast2600-mdio binding

The AST2600 splits out the MDIO bus controller from the MAC into its own
IP block and rearranges the register layout. Add a new binding to
describe the new hardware.
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/aspeed,ast2600-mdio.yaml

+# SPDX-License-Identifier: GPL-2.0-or-later
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/net/aspeed,ast2600-mdio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ASPEED AST2600 MDIO Controller
+
+maintainers:
+  - Andrew Jeffery <andrew@aj.id.au>
+
+description: |+
+  The ASPEED AST2600 MDIO controller is the third iteration of ASPEED's MDIO
+  bus register interface, this time also separating out the controller from the
+  MAC.
+
+allOf:
+  - $ref: "mdio.yaml#"
+
+properties:
+  compatible:
+    const: aspeed,ast2600-mdio
+  reg:
+    maxItems: 1
+    description: The register range of the MDIO controller instance
+
+required:
+  - compatible
+  - reg
+  - "#address-cells"
+  - "#size-cells"
+
+examples:
+  - |
+    mdio0: mdio@1e650000 {
+            compatible = "aspeed,ast2600-mdio";
+            reg = <0x1e650000 0x8>;
+            #address-cells = <1>;
+            #size-cells = <0>;
+
+            ethphy0: ethernet-phy@0 {
+                    compatible = "ethernet-phy-ieee802.3-c22";
+                    reg = <0>;
+            };
+    };