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/ti/beagle/beagle.c b/board/ti/beagle/beagle.c
index 0138fc9..12e657c 100644
--- a/board/ti/beagle/beagle.c
+++ b/board/ti/beagle/beagle.c
@@ -16,6 +16,7 @@
 #include <dm.h>
 #include <env.h>
 #include <ns16550.h>
+#include <serial.h>
 #ifdef CONFIG_LED_STATUS
 #include <status_led.h>
 #endif