watchdog: Add a flag to indicate the watchdog doesn't reboot things

Some watchdogs merely trigger external alarms and controls. In a managed
environment this is very useful but we want drivers to be able to figure
out which is which now multiple dogs can be loaded. Thus add an ALARMONLY
feature flag.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>