MAINTAINERS: Add thunderbolt.rst to the Thunderbolt driver entry

Make sure Thunderbolt maintainers get to see patches that touch
documentation of the Thunderbolt driver as well.

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