watchdog: pm8916_wdt: Report reboot reason

The PM8916 PMIC provides "power-off reason" (POFF_REASON) registers
to allow detecting why the board was powered off or rebooted. This
can be used to expose if a reset happened due to a watchdog timeout.
The watchdog API also provides status bits for overtemperature and
undervoltage which happen to be reported in the same PMIC register.

Make this information available as part of the watchdog device
so userspace can decide to handle the situation accordingly.

Signed-off-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220629084816.125515-3-stephan.gerhold@kernkonzept.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>