common: Move clock functions into a new file

These three clock functions don't use driver model and should be migrated.
In the meantime, create a new file to hold them.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/freescale/t102xrdb/spl.c b/board/freescale/t102xrdb/spl.c
index 0936f28..aed0721 100644
--- a/board/freescale/t102xrdb/spl.c
+++ b/board/freescale/t102xrdb/spl.c
@@ -3,6 +3,7 @@
  */
 
 #include <common.h>
+#include <clock_legacy.h>
 #include <console.h>
 #include <env_internal.h>
 #include <init.h>