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/common/spl/spl.c b/common/spl/spl.c
index a2ef13a..82fd54c 100644
--- a/common/spl/spl.c
+++ b/common/spl/spl.c
@@ -11,6 +11,7 @@
 #include <binman_sym.h>
 #include <dm.h>
 #include <handoff.h>
+#include <serial.h>
 #include <spl.h>
 #include <asm/u-boot.h>
 #include <nand.h>