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/doc/README.watchdog b/doc/README.watchdog
index 33f31c2..59f306b 100644
--- a/doc/README.watchdog
+++ b/doc/README.watchdog
@@ -30,3 +30,6 @@
 
 CONFIG_XILINX_TB_WATCHDOG
 	Available for Xilinx Axi platforms to service timebase watchdog timer.
+
+CONFIG_BFIN_WATCHDOG
+	Available for bf5xx and bf6xx to service the watchdog.