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/mips/mach-ath79/qca953x/clk.c b/arch/mips/mach-ath79/qca953x/clk.c
index 7447ade..d450ad8 100644
--- a/arch/mips/mach-ath79/qca953x/clk.c
+++ b/arch/mips/mach-ath79/qca953x/clk.c
@@ -4,6 +4,7 @@
  */
 
 #include <common.h>
+#include <clock_legacy.h>
 #include <asm/io.h>
 #include <asm/addrspace.h>
 #include <asm/types.h>