watchdog: qcom: Report reboot reason

The Qualcom watchdog timer block reports if the system was reset by the
watchdog. Pass the information to user space.

Reviewed-by: Grant Grundler <grundler@chromium.org>
Tested-by: Grant Grundler <grundler@chromium.org>
Signed-off-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>