blackfin: Move blackfin watchdog driver out of the blackfin arch folder.

- Enable hw_watchdog_init() in watchdog.h if CONFIG_HW_WATCHDOG is defined.
- Move blackfin hw watchdog driver to the generic driver folder.
- Call hw_watchdog_init() from blackfin board init code.
- Reuse macro CONFIG_WATCHDOG_TIMEOUT_MSECS
- Update README.watchdog accordingly

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>