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>