MAINTAINERS: Add entry for Thunderbolt network driver

I will be maintaining the Thunderbolt network driver along with Michael
and Yehezkel.

Signed-off-by: Michael Jamet <michael.jamet@intel.com>
Signed-off-by: Yehezkel Bernat <yehezkel.bernat@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>