nl802154: add support for security layer

This patch adds support for accessing mac802154 llsec implementation
over nl802154. I added for a new Kconfig entry to provide this
functionality CONFIG_IEEE802154_NL802154_EXPERIMENTAL. This interface is
still in development. It provides to change security parameters and
add/del/dump entries of security tables. Later we can add also a get to
get an entry by unique identifier.

Cc: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
diff --git a/net/ieee802154/core.h b/net/ieee802154/core.h
index f3e9558..231fade 100644
--- a/net/ieee802154/core.h
+++ b/net/ieee802154/core.h
@@ -42,5 +42,6 @@
 void cfg802154_dev_free(struct cfg802154_registered_device *rdev);
 struct cfg802154_registered_device *
 cfg802154_rdev_by_wpan_phy_idx(int wpan_phy_idx);
+struct wpan_phy *wpan_phy_idx_to_wpan_phy(int wpan_phy_idx);
 
 #endif /* __IEEE802154_CORE_H */