net/mlx5: add Rx and Tx tuning parameters

net/mlx5: add Rx and Tx tuning parameters

A new ethdev API was exposed by
commit 3be82f5cc5e3 ("ethdev: support PMD-tuned Tx/Rx parameters")

Enabling the PMD to provide default parameters in case no strict request
from application in order to improve the out of the box experience.

While the current API lacks the means for the PMD to provide the best
possible value, providing the best default the PMD can guess.
The values are based on Mellanox performance report and depends on the
underlying NIC capabilities.

Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>