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

Cc: Simon Arlott <simon@arlott.org>
Cc: Kapil Hali <kapilh@broadcom.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>