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/arch/arm/mach-imx/mx7ulp/clock.c b/arch/arm/mach-imx/mx7ulp/clock.c
index d3365dd..3c0bccc 100644
--- a/arch/arm/mach-imx/mx7ulp/clock.c
+++ b/arch/arm/mach-imx/mx7ulp/clock.c
@@ -4,6 +4,7 @@
  */
 
 #include <common.h>
+#include <clock_legacy.h>
 #include <div64.h>
 #include <asm/io.h>
 #include <errno.h>