mt76: mt7921: Add AP mode support

add AP mode support to mt7921 that can work for mt7921[e,s,u]
with the common code.

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