platform/chrome: chromeos_tbmc - fix SPDX identifier

Original submission was GPLv2 only, so mark as GPL-2.0.
Also restored some descriptive lines that were there before.

Signed-off-by: Benson Leung <bleung@chromium.org>

drivers/platform/chrome/chromeos_tbmc.c

@@ -1,8 +1,16 @@
-// SPDX-License-Identifier: GPL-2.0+
+// SPDX-License-Identifier: GPL-2.0
 // Driver to detect Tablet Mode for ChromeOS convertible.
 //
 // Copyright (C) 2017 Google, Inc.
 // Author: Gwendal Grignou <gwendal@chromium.org>
+//
+// On Chromebook using ACPI, this device listens for notification
+// from GOOG0006 and issue method TBMC to retrieve the status.
+//
+// GOOG0006 issues the notification when it receives EC_HOST_EVENT_MODE_CHANGE
+// from the EC.
+// Method TBMC reads EC_ACPI_MEM_DEVICE_ORIENTATION byte from the shared
+// memory region.
 
 #include <linux/acpi.h>
 #include <linux/input.h>