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/liebherr/display5/spl.c b/board/liebherr/display5/spl.c
index 311edaf..e845edf 100644
--- a/board/liebherr/display5/spl.c
+++ b/board/liebherr/display5/spl.c
@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <env.h>
+#include <serial.h>
 #include <spl.h>
 #include <linux/libfdt.h>
 #include <asm/io.h>