Add support for fast unload in shutdown flow in mlx5core.

This patch accumulates the following Linux commits:

• 8812c24d28f4972c4f2b9998bf30b1f2a1b62adf net/mlx5: Add fast unload support in shutdown flow
• 59211bd3b6329c3e5f4a90ac3d7f87ffa7867073 net/mlx5: Split the load/unload flow into hardware and software flows
• 4525abeaae54560254a1bb8970b3d4c225d32ef4 net/mlx5: Expose command polling interface

Submitted by: Matthew Finlay <matt@mellanox.com>
MFC after: 1 week
Sponsored by: Mellanox Technologies