mt76: mt7915: add 6 GHz support

Basic 6 GHz support is added to mt76.
Modification includes:
1. Add 6 GHz E2P definition
2. Register 6 GHz HE cap
3. Refactor existing code of adding a STA

This adds support for Wi-Fi 6E on MT7986/MT7916.
Detailed link:
https://www.mediatek.com/products/mediatek-filogic-830
Detailed link:
https://www.mediatek.com/products/products/broadband-wifi/mediatek-filogic-630

Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Shayne Chen <Shayne.Chen@mediatek.com>
Signed-off-by: Peter Chiu <Chui-Hao.Chiu@mediatek.com>
Signed-off-by: Money Wang <Money.Wang@mediatek.com>
Signed-off-by: MeiChia Chiu <MeiChia.Chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>