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

Cc: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>