mt76: mt7921: add coredump support

Introduce coredump support to mt7921 driver.

The coredump would be produced when MCU met the fatal error or driver
sent out the specific cmd to force trigger it.

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