net/mlx5: support Rx descriptor threshold event

net/mlx5: support Rx descriptor threshold event

Add mlx5 specific available descriptor threshold configuration
and query handler.
In mlx5 PMD, available descriptor threshold is also called
LWM (limit watermark).
While the Rx queue fullness reaches the LWM limit, the driver catches
an HW event and invokes the user callback.
The query handler finds the next Rx queue with pending LWM event
if any, starting from the given Rx queue index.

Signed-off-by: Spike Du <spiked@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>