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>