nfp: flower: offload merge flows

A merge flow is formed from 2 sub flows. The match fields of the merge are
the same as the first sub flow that has formed it, with the actions being
a combination of the first and second sub flow. Therefore, a merge flow
should replace sub flow 1 when offloaded.

Offload valid merge flows by using a new 'flow mod' message type to
replace an existing offloaded rule. Track the deletion of sub flows that
are linked to a merge flow and revert offloaded merge rules if required.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>