thunderbolt: Add support for simple pci tunnels

A pci downstream and pci upstream port can be connected through a
tunnel. To establish the tunnel we have to setup two unidirectional
paths between the two ports.

Right now we only support paths with two hops (i.e. no chaining) and at
most one pci device per thunderbolt device.

Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>