watchdog: s3c2410_wdt: Add support for Watchdog device on Exynos7

Exynos7 SoC has a Watchdog for Atlas (A57) cores
This patch adds support for the Atlas watchdog.

Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>