mt76: mt7921: add mt7921u driver

Introduce support for MT7921U 802.11ax 2x2:2SS wireless devices.

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