mt76: mt7921: add wifisys reset support in debugfs

Introduce chip_reset knob in mt7921 debugfs to export a way to users
able to trigger wifi reset, and group the similar operations
previously defined in chip_config in the same knob.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>