drivers: Use SPDX identifiers

Convert the remaining files in src/drivers to use SPDX identifiers.

int15.h and default_brightness_levels.asl did not have license headers,
but they were both copied from other GPL2 files, so they should be under
the GPL2 as well.

ne2k.c and drm_dp_helper.h are licensed under custom BSD-like licenses
that do not have an SPDX equivalent, so they are added as exceptions
to the license header lint.

Change-Id: I87fb1c637b8d11b0463f7c19f70b847413e14aed
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41601
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/LICENSES/retained-copyrights.txt b/LICENSES/retained-copyrights.txt
index 066bf6f..bfa9ecc 100644
--- a/LICENSES/retained-copyrights.txt
+++ b/LICENSES/retained-copyrights.txt
@@ -7,6 +7,7 @@
 Copyright 2012 Red Hat Inc.
 Copyright 2013 Google Inc.
 Copyright 2014 Google Inc.
+Copyright 2014 The Chromium OS Authors. All rights reserved.
 Copyright 2015 Google Inc.
 Copyright 2015, Google Inc.
 Copyright 2016 Jonathan Neuschäfer <j.neuschaefer@gmx.net>
@@ -16,6 +17,7 @@
 Copyright 2018 Generated Code
 Copyright 2018-present Facebook, Inc.
 Copyright 2019 9Elements Agency GmbH <patrick.rudolph@9elements.com>
+Copyright 2019 The Chromium OS Authors. All rights reserved.
 Copyright (C) 2002 David S. Peterson.  All rights reserved.
 Copyright (c) 2003-2016  Cavium Inc. (support@cavium.com). All rights
 Copyright (c) 2003-2017  Cavium Inc. (support@cavium.com). All rights