[COMMON] watchdog: s3c2410_wdt: Add multistage watchdog solution

[COMMON] watchdog: s3c2410_wdt: Add multistage watchdog solution

Multistage watchdog is a debugging solution using FIQ
when dumpGPR using reset is not available.

Multistage watchdog has a following policies.

1. Multistage watchdog is big cluster watchdog.
2. When little cluster watchdog is started,
   multistage watchdog is also started.
3. However, if the panic function runs a little cluster watchdog,
   then the multistage watchdog should be turned off.

Change-Id: I0c45d07b80c3b75848b1f1bc3259dc7faf5c63da
Signed-off-by: Choi Jaehyoung <jkkkkk.choi@samsung.com>