ieee802154: add simple HardMAC driver sample

fakehard is a really simple driver implementing only necessary
callbacks and serves the role of an example of driver for HardMAC
IEEE 802.15.4 device.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Sergey Lapin <slapin@ossfans.org>
Signed-off-by: David S. Miller <davem@davemloft.net>