vdpa/mlx5: Add hardware descriptive header file

Keep all vdpa related hardware definitions in this file.

Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Link: https://lore.kernel.org/r/20200804162048.22587-10-eli@mellanox.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>