Use atomic notifier chain to fire events to mlx5 core driver
consumers (mlx5e/mlx5_ib) and provide mlx5 register/unregister notifier
API.

This API will replace the current mlx5_interface->event callback and all
the logic around it, especially the delayed events logic introduced by
commit 97834eba ("net/mlx5: Delay events till ib registration ends")

Which is not needed anymore with this new API where the mlx5 interface
can dynamically register/unregister its notifier.
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>