netdev: Adding a new netdev API to be used for offloading flows

netdev: Adding a new netdev API to be used for offloading flows

Add a new API interface for offloading dpif flows to netdev.
The API consist on the following:
  flow_put - offload a new flow
  flow_get - query an offloaded flow
  flow_del - delete an offloaded flow
  flow_flush - flush all offloaded flows
  flow_dump_* - dump all offloaded flows

In upcoming commits we will introduce an implementation of this
API for netdev-linux.

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>