MAINTAINERS: Add git tree for Thunderbolt development

I will be gathering Thunderbolt related patches to this git tree with
help of other Thunderbolt maintainers.

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