ARM: dts: s3c64xx: Enable watchdog on all S3C64xx boards

Watchdog module does not have external dependencies so it can be safely
enabled in s3c64xx.dtsi thus making it available for all S3C64xx-based
boards.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>