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>