watchdog: s3c2410_wdt: remove the global variables

This patch removes the global variables in the driver file and
group them into a structure.

Signed-off-by: Leela Krishna Amudala <l.krishna@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>