thunderbolt: Add USB4 port devices

Create devices for each USB4 port. This is needed when we add retimer
access when there is no device connected but may be useful for other
purposes too following what USB subsystem does. This exports a single
attribute "link" that shows the type of the USB4 link (or "none" if
there is no cable connected).

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>