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/arch/blackfin/lib/board.c b/arch/blackfin/lib/board.c
index 85b859d..f1d5547 100644
--- a/arch/blackfin/lib/board.c
+++ b/arch/blackfin/lib/board.c
@@ -279,9 +279,9 @@
 	dcache_enable();
 #endif
 
-#ifdef CONFIG_WATCHDOG
+#ifdef CONFIG_HW_WATCHDOG
 	serial_early_puts("Setting up external watchdog\n");
-	watchdog_init();
+	hw_watchdog_init();
 #endif
 
 #ifdef DEBUG