watchdog: s3c2410_wdt: Report when the watchdog reset the system

A good watchdog driver is supposed to report when it was responsible
for resetting the system.  Implement this for the s3c2410, at least on
exynos5250 and exynos5420 where we already have a pointer to the PMU
registers to read the information.

Note that exynos4 SoCs also provide the reset status, but providing
that is left as an exercise for future changes and is not plumbed up
in this patch series.  Also note the exynos4 SoCs don't appear to need
any PMU config, which is why this patch separates the concepts of
having PMU Registers vs. needing PMU Config.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>