ieee802154: Give the user the association list

Upon request, we must be able to provide to the user the list of
associations currently in place. Let's add a new netlink command and
attribute for this purpose.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Stefan Schmidt <stefan@datenfreihafen.org>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/linux-wpan/20230927181214.129346-12-miquel.raynal@bootlin.com