net/mlx5: add external Rx queue mapping API

External queue is a queue that has been created and managed outside the
PMD. The queues owner might use PMD to generate flow rules using these
external queues.

When the queue is created in hardware it is given an ID represented by
32 bits. In contrast, the index of the queues in PMD is represented by
16 bits. To enable the use of PMD to generate flow rules, the queue
owner must provide a mapping between the HW index and a 16-bit index
corresponding to the ethdev API.

This patch adds an API enabling to insert/cancel a mapping between HW
queue id and ethdev queue id.

Signed-off-by: Michael Baum <michaelba@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>