watchdog: ep93xx_wdt: cleanup and let the core handle the heartbeat

Cleanup this driver and remove the 200ms heartbeat timer. The core now
has the ability to handle the heartbeat.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
[groeck: Dropped 0-initialization of static variable]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>