watchdog: s3c2410: Select MFD_SYSCON on all Exynos platforms

Syscon is used not only on Exynos5 SoCs but also on Exynos3250,
Exynos4412 and ARMv8 versions (Exynos5433, Exynos7).

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