mac802154: basic ieee802.15.4 device structures

The IEEE 802.15.4 Working Group focuses on the standardization of the
bottom two layers of ISO/OSI protocol stack: Physical (PHY) and MAC.
The MAC layer provides access control to a shared channel and reliable
data delivery. The main functions performed by the MAC sublayer are:
association and disassociation, security control, optional star
network topology functions, such as beacon generation and Guaranteed
Time Slots (GTSs) management, generation of ACK frames (if used), and,
finally, application support for the two possible network topologies
described in the standard.

This is an initial commit which describes main data structures needed
for ieee802.15.4 compatible devices representation in the MAC layer.

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