dpif: Save added ports in a port map for netdev flow api use

dpif: Save added ports in a port map for netdev flow api use

To use netdev flow offloading api, dpifs needs to iterate over
added ports. This addition inserts the added dpif ports in a hash map,
The map will also be used to translate dpif ports to netdevs.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Simon Horman <simon.horman@netronome.com>