net/mlx5: support getting hairpin peer ports

net/mlx5: support getting hairpin peer ports

In real-life business, one device could be attached and detached
dynamically. The hairpin configuration of this port to/from all the
other ports should be enabled and disabled accordingly.

The RTE ethdev lib and PMD should provide this ability to get the
peer ports list in case that the application doesn't save it. It is
recommended that the size of the array to save the port IDs is as
large as the "RTE_MAX_ETHPORTS" to have the maximal capacity.

The order of the peer port IDs may be different from that during
hairpin queues set in the initialization stage. The peer port ID
could be the same as the current device port ID when the hairpin
peer ports contain itself - the single port hairpin.

The application should check the ports' status and decide if the
peer port should be bound / unbound when starting / stopping the
current device.

Signed-off-by: Bing Zhao <bingz@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>