[net80211] initial, somewhat incomplete VHT channel setup code and attach path.

This sets up:

• vht capabilities in vaps;
• calls vht_announce to announce VHT capabilities if any;
• sets up vht20, vht40 and vht80 channels, assuming the regulatory code does the right thing with 80MHz available ranges;
• adds support to the ieee80211_add_channel_list_5ghz() code to populate VHT channels, as this is the API my ath10k driver is using;
• add support for the freq1/freq2 field population and lookup that VHT channels require.

The VHT80 code assumes that the regulatory domain already has limited VHT80
bands to, well, 80MHz wide chunks.