mt76: connac: move mcu reg access utility routines in mt76_connac_lib module

Move mcu reg access shared between mt7663s and mt7921s in mt76_connac_lib
module.

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