mt76: introduce mmio data structure in mt76_dev

Introduce mt76_mmio data structure in mt76_dev and
move mt76x2_mcu in mt76_mmio. This is a preliminary
patch to unify mcu code between mt76x02{e,u} drivers

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>