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/drivers/serial/ns16550.c b/drivers/serial/ns16550.c
index 754b6e9..a92d2b1 100644
--- a/drivers/serial/ns16550.c
+++ b/drivers/serial/ns16550.c
@@ -4,6 +4,7 @@
  * modified to use CONFIG_SYS_ISA_MEM and new defines
  */
 
+#include <clock_legacy.h>
 #include <common.h>
 #include <clk.h>
 #include <dm.h>