thunderbolt: Improve logging when DisplayPort resource is added due to hotplug

To allow us differentiate how DisplayPort resource is added to the
DisplayPort resources list make the debug log to append "hotplug" when
this was due to an actual hotplug.

Signed-off-by: Gil Fine <gil.fine@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>