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>
diff --git a/drivers/clk/clk-gpio.c b/drivers/clk/clk-gpio.c
index 6a43ce4..25eed3e 100644
--- a/drivers/clk/clk-gpio.c
+++ b/drivers/clk/clk-gpio.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (C) 2013 - 2014 Texas Instruments Incorporated - http://www.ti.com
  *
@@ -5,10 +6,6 @@
  *    Jyri Sarha <jsarha@ti.com>
  *    Sergej Sawazki <ce3a@gmx.de>
  *
- * 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.
- *
  * Gpio controlled clock implementation
  */