ieee80211: add TWT element definitions

Introduce TWT definitions and TWT Information element structure
in ieee80211.h

Tested-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/71d8b581fe4b5abc5b92f8d77ac2de3e2f7591b6.1629741512.git.lorenzo@kernel.org


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