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>
diff --git a/include/watchdog.h b/include/watchdog.h
index 97ec186..d95e4b1 100644
--- a/include/watchdog.h
+++ b/include/watchdog.h
@@ -108,8 +108,7 @@
 	void reset_4xx_watchdog(void);
 #endif
 
-/* Freescale i.MX */
-#if defined(CONFIG_IMX_WATCHDOG) && !defined(__ASSEMBLY__)
+#if defined(CONFIG_HW_WATCHDOG) && !defined(__ASSEMBLY__)
 	void hw_watchdog_init(void);
 #endif
 #endif /* _WATCHDOG_H_ */