wifi: mt76: mt7921: move common register definition in mt792x_regs.h

This is a preliminary patch in order to support new WiFi7 chips.
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>