drivers/ieee802154: add support for the at86rf230/231 transceivers

The AT86RF231 is a feature rich, low-power 2.4 GHz radio transceiver
designed for industrial and consumer ZigBee/IEEE 802.15.4, 6LoWPAN,
RF4CE and high data rate 2.4 GHz ISM band applications.

This patch adds support for the Atmel RF230/231 radio transceivers.

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