6lowpan: add lowpan dev register helpers

This patch introduces register and unregister functionality for lowpan
interfaces. While register a lowpan interface there are several things
which need to be initialize by the 6lowpan subsystem. Upcoming
functionality need to register/unregister per interface components e.g.
debugfs entry.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>