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>

sound/soc/mxs/mxs-pcm.c

@@ -357,3 +357,5 @@ static void __exit snd_mxs_pcm_exit(void)
 	platform_driver_unregister(&mxs_pcm_driver);
 }
 module_exit(snd_mxs_pcm_exit);
+
+MODULE_LICENSE("GPL");