The information contained in the documentation are covered by the BSD
2-Clause license unless stated otherwise. Make this clear by not
mentioning "source files" but simply "files" in LICENSE.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Suggested-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>