watchdog: cpwd: add of_node_put()

use of_node_put() to release the refcount.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>