dt-bindings: watchdog: add i.MX system controller watchdog

Add i.MX system controller watchdog binding doc.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>