cfg80211/nl80211: Add PMKSA caching candidate event

When the driver (or most likely firmware) decides which AP to use
for roaming based on internal scan result processing, user space
needs to be notified of PMKSA caching candidates to allow RSN
pre-authentication to be used.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>