mac802154: allocation of ieee802154 device

An interface to allocate and register ieee802154 compatible device.
The allocated device has the following representation in memory:

	+-----------------------+
	| struct wpan_phy       |
	+-----------------------+
	| struct mac802154_priv |
	+-----------------------+
	| driver's private data |
	+-----------------------+

Used by device drivers to register new instance in the stack.

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