wifi: mt76: mt7915: add full system reset into debugfs

Trigger firmware crash and enable full system recovery through debugfs.
This also renames knob "fw_ser" to a clear-cut name "sys_recovery".

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>