ath10k: create debugfs interface to trigger fw crash

This can be useful for testing. To perform a
forced firmware crash write 'crash' to
'simulate_fw_crash' debugfs file. E.g.

  echo crash > /sys/kernel/debug/ieee80211/phy1/ath10k/simulate_fw_crash

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