When a watchdog timeout is set in the linux kernel, the FORCE_UPDATE bit is set.
Use this to recognize a system reset that was triggered by the watchdog.
Since only the wdt part of the kernel driver sets this bit, this is a safe
distinguishing feature.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>