dt-bindings: watchdog: move i.MX system controller watchdog binding to SCU

i.MX system controller watchdog depends on SCU driver to support
interrupt function, so it needs to be subnode of SCU node in DT,
binding doc should be moved to fsl,scu.txt as well.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
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>