arm64: dts: realtek: Add watchdog node for RTD129x

Add the watchdog node to the RTD129x Device Tree.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
[AF: Moved from RTD1295 to new RTD129x]
Signed-off-by: Andreas Färber <afaerber@suse.de>