cfg80211: support FTM responder configuration/statistics

Allow userspace to enable fine timing measurement responder
functionality with configurable lci/civic parameters in AP mode.
This can be done at AP start or changing beacon parameters.

A new EXT_FEATURE flag is introduced for drivers to advertise
the capability.

Also nl80211 API support for retrieving statistics is added.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Pradeep Kumar Chitrapu <pradeepc@codeaurora.org>
[remove unused cfg80211_ftm_responder_params, clarify docs,
 move validation into policy]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>