ieee802154: add cfg802154_registered_device list

This patch adds a new cfg802154_rdev_list to remember all registered
cfg802154_registered_device structs. This is needed to prepare the
upcomming nl802154 framework.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>