dt-bindings: net: airoha: Add airoha,npu phandle property

Introduce the airoha,npu property for the NPU node available on
EN7581 SoC. The airoha Network Processor Unit (NPU) is used to
offload network traffic forwarded between Packet Switch Engine
(PSE) ports.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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

@@ -63,6 +63,14 @@ properties:
   "#size-cells":
     const: 0
 
+  airoha,npu:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      Phandle to the node used to configure the NPU module.
+      The Airoha Network Processor Unit (NPU) provides a configuration
+      interface to implement hardware flow offloading programming Packet
+      Processor Engine (PPE) flow table.
+
 patternProperties:
   "^ethernet@[1-4]$":
     type: object
@@ -132,6 +140,8 @@ examples:
                      <GIC_SPI 49 IRQ_TYPE_LEVEL_HIGH>,
                      <GIC_SPI 64 IRQ_TYPE_LEVEL_HIGH>;
 
+        airoha,npu = <&npu>;
+
         #address-cells = <1>;
         #size-cells = <0>;