MAINTAINERS: add maintainer for generic 6LoWPAN

Add Jukka to 6LoWPAN maintainer list. He will concentrate on
generic and bluetooth part of 6LoWPAN stack.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>