mt76: mt7663u: introduce suspend/resume to mt7663u

Tested on Chromebok by "echo mem to /sys/power/state" to suspend and
then waked up by keyboard keystrokes to resume system.

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