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>

include/clock_legacy.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * (C) Copyright 2000-2009
+ * Wolfgang Denk, DENX Software Engineering, wd@denx.de.
+ */
+
+#ifndef __CLOCK_LEGACY_H
+#define __CLOCK_LEGACY_H
+
+int get_clocks(void);
+unsigned long get_bus_freq(unsigned long dummy);
+int get_serial_clock(void);
+
+#endif