mt76: mt7921: add MT7921_COMMON module

This is a preliminary patch to introduce mt7921s support.

MT7921_COMMON module grouping bus independent objects the both mt7921e and
mt7921s can share with and have to rely on.

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>