Use SPDX-License-Identifier instead of a verbose license text.

CC: Alessandro Zummo <a.zummo@towertech.it>
CC: Alexandre Belloni <alexandre.belloni@bootlin.com>
CC: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Link: https://lore.kernel.org/r/20191218082836.3309808-1-iwamatsu@nigauri.orgSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>