rtw89: support FW crash simulation

rtw89: support FW crash simulation

Originally, there is already a mechanism, SER (system error recover),
to deal with HW/FW recovery. After FW v0.13.36.0, FW supports a H2C
(host to chip) command to make a CPU exception. Then, SER is supposed
to catch this FW crash and do L2 reset. This feature is a simulation
to verify if flow of recovering from FW crash works.

Usage of fw_crash debugfs is as the following.
$ echo 1 > fw_crash	// trigger FW crash and wait SER handling
$ cat fw_crash		// return 0 if restart has been done

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220314071250.40292-9-pkshih@realtek.com