net: airoha: Introduce Airoha NPU support

Packet Processor Engine (PPE) module available on EN7581 SoC populates
the PPE table with 5-tuples flower rules learned from traffic forwarded
between the GDM ports connected to the Packet Switch Engine (PSE) module.
The airoha_eth driver can enable hw acceleration of learned 5-tuples
rules if the user configure them in netfilter flowtable (netfilter
flowtable support will be added with subsequent patches).
airoha_eth driver configures and collects data from the PPE module via a
Network Processor Unit (NPU) RISC-V module available on the EN7581 SoC.
Introduce basic support for Airoha NPU module.

Tested-by: Sayantan Nandy <sayantan.nandy@airoha.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>