net/mlx5: Add header file for events

Separate the event API defined in the generic mlx5.h header into
a dedicated header. And remove the TODO comment in commit
69c1280b ("net/mlx5: Device events, Use async events chain").

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>