net: ethernet: mtk_eth_soc: rework hardware flow table management

The hardware was designed to handle flow detection and creation of flow entries
by itself, relying on the software primarily for filling in egress routing
information.
When there is a hash collision between multiple flows, this allows the hardware
to maintain the entry for the most active flow.
Additionally, the hardware only keeps offloading active for entries with at
least 30 packets per second.

With this rework, the code no longer creates a hardware entries directly.
Instead, the hardware entry is only created when the PPE reports a matching
unbound flow with the minimum target rate.
In order to reduce CPU overhead, looking for flows belonging to a hash entry
is rate limited to once every 100ms.

This rework is also used as preparation for emulating bridge offload by
managing L4 offload entries on demand.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: David S. Miller <davem@davemloft.net>