ti: Add SPDX license identifier to omap.h

This also came from Linux - according to this thread it has a GPL v2
license like arch/arm/mach-omap2/mux.h:

http://lists.denx.de/pipermail/u-boot/2015-June/217827.html

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Ingrid Viitanen <ingrid.viitanen@nokia.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

include/dt-bindings/pinctrl/omap.h

@@ -3,6 +3,8 @@
  *
  * Copyright (C) 2009 Nokia
  * Copyright (C) 2009-2010 Texas Instruments
+ *
+ * SPDX-License-Identifier:	GPL-2.0
  */
 
 #ifndef _DT_BINDINGS_PINCTRL_OMAP_H