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/serial_ns16550.c b/drivers/serial/serial_ns16550.c
index a1069f9..ef394b7 100644
--- a/drivers/serial/serial_ns16550.c
+++ b/drivers/serial/serial_ns16550.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <clock_legacy.h>
 #include <ns16550.h>
 #include <serial.h>
 #include <linux/compiler.h>