ASoC: mxs: Add missing MODULE_LICENSE("GPL")

The sound driver refuses to load as module, because of the missing
MODULE_LICENSE("GPL").
The file header indicates that the driver is indeed published under
the GPL.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>