mt76: add mac80211 driver for MT7915 PCIe-based chipsets

Add support for the MediaTek latest generation IEEE 802.11ax 4x4
device MT7915E, which supports concurrent dual-band operation at
both 5GHz and 2.4GHz.

Note that this patch just add basic part and will add more HE
capabilities support in the further patches.

The driver supports AP, Station, Mesh and monitor mode.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Chih-Min Chen <chih-min.chen@mediatek.com>
Suggested-by: Shihwei Lin <shihwei.lin@mediatek.com>
Tested-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Acked-by: Yiwei Chung <yiwei.chung@mediatek.com>
Acked-by: YF Luo <yf.luo@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>