mt76: add mac80211 driver for MT7615 PCIe-based chipsets

This driver is for a newer generation of MediaTek MT7615 4x4 802.11ac
PCIe-based chipsets, which support wave2 MU-MIMO up to 4 users/group
and also support up to 160MHz bandwidth.

The driver fully supports AP, station and monitor mode.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Roy Luo <royluo@google.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>