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>

include/configs/bfin_adi_common.h

@@ -314,5 +314,11 @@
 #define CONFIG_BFIN_SPI_GPIO_CS /* Only matters if BFIN_SPI is enabled */
 #define CONFIG_LZMA
 #define CONFIG_MONITOR_IS_IN_RAM
+#ifdef CONFIG_HW_WATCHDOG
+# define CONFIG_BFIN_WATCHDOG
+# ifndef CONFIG_WATCHDOG_TIMEOUT_MSECS
+#  define CONFIG_WATCHDOG_TIMEOUT_MSECS 5000
+# endif
+#endif
 
 #endif