wifi_mgmt: Add support for frequency bands

Channels are not unique across bands, they need to be interpreted along
with frequency band. Add a separate band parameter that supports 2.4, 5
and 6 GHz bands.

Also, 6GHz has maximum channel of 233 (as of now), so, change the
maximum channel to that, this is a global maximum, per-band channel
validation is left to the offload driver as that depends on regulatory
rules (not a simple maximum check).

Signed-off-by: Krishna T <krishna.t@nordicsemi.no>