mt76: mt7921: add MCU support

MT7921 contains a microprocessor with which the host can use command/event
to communicate to implement offload features such as establish connection,
hardware scan and so on. The host has to download the ROM patch, RAM
firmware and finally activate the MCU to complete the MT7921
initialization.

Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Co-developed-by: Soul Huang <Soul.Huang@mediatek.com>
Signed-off-by: Soul Huang <Soul.Huang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>