dt-bindings: net: airoha: Add GDM port ethernet child node

EN7581 and AN7583 SoCs support connecting multiple external SerDes to GDM3
or GDM4 ports via a hw arbiter that manages the traffic in a TDM manner.
As a result multiple net_devices can connect to the same GDM{3,4} port
and there is a theoretical "1:n" relation between GDM ports and
net_devices.
Introduce the ethernet node child of a specific GDM port in order to model
a given net_device that is connected via the external arbiter to the
GDM{3,4} port. This new ethernet node is defined by the "airoha,eth-port"
compatible string. Please note GDM1 and GDM2 does not support the
connection with the external arbiter and they are represented by an
ethernet node defined by the "airoha,eth-mac" compatible string.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260603-airoha-eth-multi-serdes-v9-1-5d476bc2f426@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/airoha,en7581-eth.yaml

@@ -130,6 +130,42 @@ patternProperties:
         maximum: 4
         description: GMAC port identifier
 
+    allOf:
+      - if:
+          properties:
+            reg:
+              contains:
+                items:
+                  - enum:
+                      - 3
+                      - 4
+        then:
+          properties:
+            '#address-cells':
+              const: 1
+
+            '#size-cells':
+              const: 0
+
+          patternProperties:
+            "^ethernet@[0-5]$":
+              type: object
+              unevaluatedProperties: false
+              $ref: ethernet-controller.yaml#
+              description: External ethernet port ID available on the GDM port
+
+              properties:
+                compatible:
+                  const: airoha,eth-port
+
+                reg:
+                  maximum: 5
+                  description: External ethernet port identifier
+
+              required:
+                - reg
+                - compatible
+
     required:
       - reg
       - compatible
@@ -191,9 +227,27 @@ examples:
         #address-cells = <1>;
         #size-cells = <0>;
 
-        mac: ethernet@1 {
+        ethernet@1 {
           compatible = "airoha,eth-mac";
           reg = <1>;
         };
+
+        ethernet@4 {
+          compatible = "airoha,eth-mac";
+          reg = <4>;
+
+          #address-cells = <1>;
+          #size-cells = <0>;
+
+          ethernet@0 {
+            compatible = "airoha,eth-port";
+            reg = <0>;
+          };
+
+          ethernet@1 {
+            compatible = "airoha,eth-port";
+            reg = <1>;
+          };
+        };
       };
     };