ethdev: add represented port item to flow API

ethdev: add represented port item to flow API

For use in "transfer" flows. Supposed to match traffic entering the
embedded switch from the entity represented by the given ethdev.
Such an entity can be a network (via a network port), a guest
machine (via a VF) or another ethdev in the same application.

Must not be combined with direction attributes.

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Acked-by: Ori Kam <orika@nvidia.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>