iwlwifi: initial P2P support

If PAN functionality is present, advertise
P2P interface type support and thus support
for P2P. However, the support is currently
somewhat incomplete -- NoA schedule isn't
added to probe responses, and 11b bitrates
may be used still, etc. Therefore, make it
all optional with a Kconfig option.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>