ASoC: ep93xx-pcm: add MODULE_ALIAS

To get the PCM module loaded automatically by udev et al. we need to add a
proper MODULE_ALIAS.

Signed-off-by: Mika Westerberg <mika.westerberg@iki.fi>
Reviewed-by: Ryan Mallon <rmallon@gmail.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>