mac802154: move interface add handling in iface

This patch moves and renames the mac802154_add_iface and
mac802154_netdev_register functions into iface.c. The function
mac802154_add_iface is renamed to ieee802154_if_add which is a similar naming
convention like mac80211.

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