TODO: change WDOG_KEEP_ON to autostart module parameter

changelog:
[PATCH 1/6] watchdog: watchdog_dev: WATCHDOG_KEEP_ON feature
* clean up old comments
* watchdog_keepon_start() renamed to watchdog_timer_start()
* watchdog_keepon_stop() renamed to watchdog_timer_stop()
* watchdog_timer_register() added
* watchdog_timer_unregister() added
* watchdog_timer_register() is the last action
  in watchdog_dev_register().
  FIXME: Really should be in probe()?
* TODO: should watchdog_timer_start() call watchdog_start()?
* watchdog_release() uses watchdog_timer_restart() helper function