mt76: introduce struct mt76_phy

This is preparation for supporting multiple wiphys per device to support the
concurrent dual-band feature of MT7615D

On the first wiphy, hw->priv will point to struct mt76_dev, which contains a
struct mt76_phy at the start. For the secondary wiphy, hw->priv will point
to a mt76_phy encapsulated in a driver specific struct

To simplify access to struct mt76_phy members from drivers, the driver specific
device struct is changed to add a union of struct mt76_dev and struct mt76_phy
as the first element

Signed-off-by: Felix Fietkau <nbd@nbd.name>