Wifi: Add 6GHz bands to WifiBand

This commit adds 6GHz band to WifiBand and update the impacted
structures and methods in Wifi HAL.
Note that Background scar related HAL API will not be using the
new band, and will continue with the legacy set of bands.

Bug: 139354972
Test: Manual
Test: VTS test
Test: Unit test: ./hardware/interfaces/wifi/1.4/default/tests/runtests.sh
Change-Id: I54662251034806338ad64d4622ade259ca260a79