staging: mt7621-dma: add SPDX GPL-2.0+ license identifier

Add SPDX GPL-2.0+ license to fix missing SPDX warning
 reported by checkpatch.pl.

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>