cfg802154: introduce cfg802154_registered_device

This patch introduce the cfg802154_registered_device struct. Like
cfg80211_registered_device in wireless this should contain similar
functionality for cfg802154. This patch should not change any behaviour.
We just adds cfg802154_registered_device as container for wpan_phy struct.

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