mt76: add 6GHz support

Introduce 6GHz channel list in mt76 module. This is a preliminary patch
to unlock 6GHz band for mt7921 devices.

Tested-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>