cfg80211: allow requesting SMPS mode on ap start

Add feature bits to indicate device support for
static-smps and dynamic-smps modes.

Add a new NL80211_ATTR_SMPS_MODE attribue to allow
configuring the smps mode to be used by the ap
(e.g. configuring to ap to dynamic smps mode will
reduce power consumption while having minor effect
on throughput)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>