thunderbolt: Add path setup code.

A thunderbolt path is a unidirectional channel between two thunderbolt
ports. Two such paths are needed to establish a pci tunnel.

This patch introduces struct tb_path as well as a set of tb_path_*
methods which are used to activate & deactivate paths.

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