mac80211_hwsim: Add iface comb for DFS

Add iface combination that will allow DFS
support. Add also debugfs dfs_simulate_radar
file that can be used to simulate radar event.
This could be useful for mac80211/cfg80211/
regulatory/hostap code testing without real HW.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>