nfp: add a stats handler for flower offloads

Previously there was no way of updating flow rule stats after they
have been offloaded to hardware. This is solved by keeping track of
stats received from hardware and providing this to the TC handler
on request.

Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>