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/socrates/socrates.c b/board/socrates/socrates.c
index f1dec6c..b0ddee7 100644
--- a/board/socrates/socrates.c
+++ b/board/socrates/socrates.c
@@ -11,6 +11,7 @@
  */
 
 #include <common.h>
+#include <clock_legacy.h>
 #include <env.h>
 #include <pci.h>
 #include <asm/processor.h>