net: airoha: npu: Add a NPU callback to initialize flow stats

net: airoha: npu: Add a NPU callback to initialize flow stats

Introduce a NPU callback to initialize flow stats and remove NPU stats
initialization from airoha_npu_get routine. Add num_stats_entries to
airoha_npu_ppe_stats_setup routine.
This patch makes the code more readable since NPU statistic are now
initialized on demand by the NPU consumer (at the moment NPU statistic
are configured just by the airoha_eth driver).
Moreover this patch allows the NPU consumer (PPE module) to explicitly
enable/disable NPU flow stats.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250924-airoha-npu-init-stats-callback-v1-1-88bdf3c941b2@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>