wifi: mt76: mt7921: introduce chanctx support

The firmware can have the capability to manage the channel context
scheduling on multiple roles running on the device including Station,
AP and P2P GC/GO mode (will be extended based on the future patchset)
to help users sharing the network with others on a single device.

The firmware is able to support the channel chanctx up to 2 interface
simultaneously running on the different channels.

Another thing to be noted is that before the driver is going sent out the
management frames, the driver has to get the privilege from the firmware
to occupy the current channel context until the frame handshake is
completed and then get the privilege back to the firmware.

Co-developed-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>