ieee80211: add HE ext EIDs and 6 GHz capability defines

Add the HE extended element IDs and the definitions for the
HE 6 GHz band capabilities element, from Draft 5.0.

Link: https://lore.kernel.org/r/20200528213443.1a6689fe093f.Ifdc5400fb01779351354daf38663ebeea03c9ad9@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>