cfg80211: allow userspace to specify client P2P PS support

Legacy clients don't support P2P power save mechanisms, and thus
if a P2P GO has a legacy client connected to it, it has to make
some changes in the PS behavior.

To handle this, add an attribute to specify whether a station supports
P2P PS or not. If the attribute was not specified cfg80211 will assume
that station supports it for P2P GO interface, and does NOT support it
for AP interface, matching the current assumptions in the code.

Signed-off-by: Ayala Beker <ayala.beker@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>