[lib80211] add VHT bands and channel flags.

This is preparation work for 11ac support. The regulatory database
needs to know about VHT channel flags and 80MHz (and later 160MHz)
available channel bands.

Whilst here, add the 2GHz VHT band (which is a terrible, terrible vendor
extension that almost all vendors do) just in preparation, even though
I don't (yet) plan on supporting it.