ath10k: expose hw restart via debugfs

Until now it was possible to simulate soft and
hard fw crashes but it wasn't possible to trigger
an immediately hw restart itself (without the fw
crash).

This can be useful when stress testing hw
restarting stability, e.g. during heavy tx/rx
traffic.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>