[COMMON] watchdog: s3c2410_wdt: add stutdown disable

[COMMON] watchdog: s3c2410_wdt: add stutdown disable

- When system failure after watchdog driver shutdown,
  there is no way to reboot, just system hang (with black screen).
- To prevent system hang, watchdog must not be disabled after shutdown.
- But sometimes watchdog must be disabled to debug reset sequence.
  So, define with config : S3C2410_SHUTDOWN_REBOOT

- This configuration is recommended for user case.

Change-Id: I2cfcc6d922f2ccd8cd91d0059fb6f79e9fd40854
Signed-off-by: hajun.sung <hajun.sung@samsung.com>