src/acpi: Update license headers to SPDX

While I was working on updating the headers to move copyrights into
the AUTHORS file, I got a request to switch to SPDX headers as well.

Linux has moved completely to SPDX headers, which are easier to
maintain, have good definitions, are very short, and can be checked
automatically.  This is completely unlike our current header situation.

Signed-off-by: Martin Roth <martin@coreboot.org>
Change-Id: Ie86d34f7fa7bf7434ad8a38aa1eadcfece7124b3
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36176
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
diff --git a/src/acpi/Makefile.inc b/src/acpi/Makefile.inc
index 53ac679..7c2092d 100644
--- a/src/acpi/Makefile.inc
+++ b/src/acpi/Makefile.inc
@@ -1 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
+# This file is part of the coreboot project.
+
 ramstage-$(CONFIG_ACPI_SATA_GENERATOR) += sata.c