ASoC: pcm512x: add missing MODULE_DESCRIPTION/AUTHOR/LICENSE

This change resolves a new compile-time warning
when built as a loadable module:

WARNING: modpost: missing MODULE_LICENSE() in sound/soc/codecs/snd-soc-pcm512x-spi.o
see include/linux/module.h for more information

This adds the license as "GPL v2", which matches the header of the file.

MODULE_DESCRIPTION and MODULE_AUTHOR are also added.

Signed-off-by: Jesse Chan <jc@linux.com>
Signed-off-by: Mark Brown <broonie@kernel.org>