blackfin: Move blackfin serial driver out of blackfin arch folder.

- Move blackfin serial driver to the generic driver folder.
- Move blackfin serial headers to blackfin arch head folder.
- Update the include path to blackfin serial header in start up code.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/arch/blackfin/cpu/start.S b/arch/blackfin/cpu/start.S
index 1c6ae35..da084a8 100644
--- a/arch/blackfin/cpu/start.S
+++ b/arch/blackfin/cpu/start.S
@@ -35,8 +35,7 @@
 #include <asm/mach-common/bits/watchdog.h>
 #include <asm/mach-common/bits/core.h>
 #include <asm/mach-common/bits/pll.h>
-
-#include "serial.h"
+#include <asm/serial.h>
 
 /* It may seem odd that we make calls to functions even though we haven't
  * relocated ourselves yet out of {flash,ram,wherever}.  This is OK because