drivers: Add SPDX license headers to Makefiles

To help identify the licenses of the various files contained in the
coreboot source, we've added SPDX headers to the top of all of the
.c and .h files. This extends that practice to Makefiles.

Any file in the coreboot project without a specific license is bound
to the license of the overall coreboot project, GPL Version 2.

This patch adds the GPL V2 license identifier to the top of all
makefiles in the drivers directory that don't already have an SPDX
license line at the top.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I8442bc18ce228eca88a084660be84bcd1c5de928
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68980
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Reviewed-by: Tim Crawford <tcrawford@system76.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

src/drivers/sof/Makefile.inc

@@ -1 +1,3 @@
+## SPDX-License-Identifier: GPL-2.0-only
+
 ramstage-$(CONFIG_DRIVERS_AUDIO_SOF) += sof.c