MAINTAINERS: add net/6lowpan/ maintainer entry

This patch add a maintainer entry for "net/6lowpan". Also add the current
IEEE 802.15.4 mailing list and bluetooth mailinglist to this branch,
because this code is shared between them.

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