fbdev: imxfb: add missing SPDX tag

Resolve the following warning reported by checkpatch.pl:

WARNING: Missing or malformed SPDX-License-Identifier tag in line 1

The patch also removes some license info made redundant by the addition
of the SPDX tag.
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Helge Deller <deller@gmx.de>

drivers/video/fbdev/imxfb.c

+// SPDX-License-Identifier: GPL-2.0-only
 /*
  *  Freescale i.MX Frame Buffer device driver
  *
  *  Copyright (C) 2004 Sascha Hauer, Pengutronix
  *   Based on acornfb.c Copyright (C) Russell King.
  *
- * This file is subject to the terms and conditions of the GNU General Public
- * License.  See the file COPYING in the main directory of this archive for
- * more details.
- *
  * Please direct your questions and comments on this driver to the following
  * email address:
  *