ARM: dts: exynos: Enable watchdog on all Exynos4 boards

Watchdog module does not have external dependencies so it can be safely
enabled in exynos4.dtsi thus making it available for all Exynos4-based
boards.

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