devlink: add a file documenting devlink regions

Also document the regions created by the mlx4 driver. This is currently
the only in-tree driver that creates devlink region snapshots.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>