cfg80211: Add AP stopped interface

AP stopped interface can be used to indicate that the AP mode has
stopped functioning, WLAN driver may have encountered errors that has
forced the driver to stop the AP mode.

When the driver is in P2P-Go mode, and when it goes thru automatic
recovery from firmware crashes, it uses this interface to notify the
userspace that the group has been deleted.

CRs-Fixed: 1078172
Change-Id: Ifcd8d4f0c0b26f56a56fb8560aa474297b7521d4
Signed-off-by: Sameer Thalappil <sameert@codeaurora.org>