clk: Tag basic clk types with SPDX

These are all GPL-2.0 files per the existing license text. Replace the
boiler plate with the tag.

Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/clk-mux.c

@@ -1,12 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (C) 2011 Sascha Hauer, Pengutronix <s.hauer@pengutronix.de>
  * Copyright (C) 2011 Richard Zhao, Linaro <richard.zhao@linaro.org>
  * Copyright (C) 2011-2012 Mike Turquette, Linaro Ltd <mturquette@linaro.org>
  *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- *
  * Simple multiplexer clock implementation
  */