ASoC: pxa: Fix module autoload for platform drivers

These platform drivers are lacking MODULE_ALIAS so module autoloading
doesn't work. Tested on corgi and poodle with kernel 4.4.

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/sound/soc/pxa/mmp-pcm.c b/sound/soc/pxa/mmp-pcm.c
index 51e790d..96df9b2 100644
--- a/sound/soc/pxa/mmp-pcm.c
+++ b/sound/soc/pxa/mmp-pcm.c
@@ -248,3 +248,4 @@
 MODULE_AUTHOR("Leo Yan <leoy@marvell.com>");
 MODULE_DESCRIPTION("MMP Soc Audio DMA module");
 MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:mmp-pcm-audio");