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>