watchdog: wdat_wdt: Stop watchdog when rebooting the system

Executing reboot command several times on the machine "Dell
PowerEdge R740", UEFI security detection stopped machine
with the following prompt:

UEFI0082: The system was reset due to a timeout from the watchdog
timer. Check the System Event Log (SEL) or crash dumps from
Operating Sysstem to identify the source that triggered the
watchdog timer reset. Update the firmware or driver for the
identified device.

iDRAC has warning event: "The watchdog timer reset the system".

This patch fixes this issue by adding the reboot notifier.

Signed-off-by: Liu Xinpeng <liuxp11@chinatelecom.cn>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/1650984810-6247-3-git-send-email-liuxp11@chinatelecom.cn
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>