mac802154: slave interfaces declaration

Slaves represent typical network interfaces available from userspace.
Each ieee802154 device/transceiver may have several slaves and able
to be associated with several networks at the same time. So this
patch adds structure for slaves declaration.

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>