net: airoha: Add FLOW_CLS_STATS callback support

net: airoha: Add FLOW_CLS_STATS callback support

Introduce per-flow stats accounting to the flowtable hw offload in
the airoha_eth driver. Flow stats are split in the PPE and NPU modules:
- PPE: accounts for high 32bit of per-flow stats
- NPU: accounts for low 32bit of per-flow stats

FLOW_CLS_STATS can be enabled or disabled at compile time.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250516-airoha-en7581-flowstats-v2-2-06d5fbf28984@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>