dt-bindings: net: airoha: Add the NPU node for EN7581 SoC

This patch adds the NPU document binding for EN7581 SoC.
The Airoha Network Processor Unit (NPU) provides a configuration interface
to implement wired and wireless hardware flow offloading programming Packet
Processor Engine (PPE) flow table.

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