nfp: flower: add infastructure for non-repr priv data

NFP repr netdevs contain private data that can store per port information.
In certain cases, the NFP driver offloads information from non-repr ports
(e.g. tunnel ports). As the driver does not have control over non-repr
netdevs, it cannot add/track private data directly to the netdev struct.

Add infastructure to store private information on any non-repr netdev that
is offloaded at a given time. This is used in a following patch to track
offloaded MAC addresses for non-reprs and enable correct house keeping on
address changes.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>