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/clk/mpc83xx_clk.c b/drivers/clk/mpc83xx_clk.c
index 69c6207..4183db2 100644
--- a/drivers/clk/mpc83xx_clk.c
+++ b/drivers/clk/mpc83xx_clk.c
@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <clk-uclass.h>
+#include <clock_legacy.h>
 #include <dm.h>
 #include <vsprintf.h>
 #include <dm/lists.h>