thunderbolt: Introduce USB only (SL4) security level

This new security level works so that it creates one PCIe tunnel to the
connected Thunderbolt dock, removing PCIe links downstream of the dock.
This leaves only the internal USB controller visible.

Display Port tunnels are created normally.

While there make sure security sysfs attribute returns "unknown" for any
future security level.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>