nfp: flower: handle merge hint messages

If a merge hint is received containing 2 flows that are matched via an
implicit recirculation (sending to and matching on an internal port), fw
reports that the flows (called sub_flows) may be able to be combined to a
single flow.

Add infastructure to accept and process merge hint messages. The actual
merging of the flows is left as a stub call.

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>