Adopt the SPDX license identifier headers to ease license compliance
management.

Cc: Simon Arlott <simon@arlott.org>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: NStefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: NEric Anholt <eric@anholt.net>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>