nfp: flower: support stats update for merge flows

With the merging of 2 sub flows, a new 'merge' flow will be created and
written to FW. The TC layer is unaware that the merge flow exists and will
request stats from the sub flows. Conversely, the FW treats a merge rule
the same as any other rule and sends stats updates to the NFP driver.

Add links between merge flows and their sub flows. Use these links to pass
merge flow stats updates from FW to the underlying sub flows, ensuring TC
stats requests are handled correctly. The updating of sub flow stats is
done on (the less time critcal) TC stats requests rather than on FW stats
update.

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>