ieee802154: add support for listen-before-talk in wpan_phy

Listen-before-talk is an alternative to CSMA in uncoordinated networks
and prescribed by european regulations if one wants to have a device
with radio duty cycles above 10% (or less in some bands). Add a phy
property to enable/disable LBT in the phy, including support in the
at86rf230 driver for RF212 chips.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/nl802154.h

@@ -71,6 +71,7 @@ enum {
 	IEEE802154_ATTR_DEV_TYPE,
 
 	IEEE802154_ATTR_TXPOWER,
+	IEEE802154_ATTR_LBT_ENABLED,
 
 	__IEEE802154_ATTR_MAX,
 };