common: Move serial functions out of common.h

These functions belong in serial.h so move them over.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/board/siemens/common/board.c b/board/siemens/common/board.c
index 75462d1..5f5e2eb 100644
--- a/board/siemens/common/board.c
+++ b/board/siemens/common/board.c
@@ -12,6 +12,7 @@
 #include <common.h>
 #include <env.h>
 #include <errno.h>
+#include <serial.h>
 #include <spl.h>
 #include <asm/arch/cpu.h>
 #include <asm/arch/hardware.h>