ieee802154: Add support for user association requests

Users may decide to associate with a peer, which becomes our parent
coordinator. Let's add the necessary netlink support for this.

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/20230927181214.129346-4-miquel.raynal@bootlin.com