watchdog: s3c2410: convert to use the watchdog framework

Make this driver a user of the watchdog framework and remove now
centrally handled parts. Tested on a mini2440.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>