ASoC: pcm179x: Split into core and SPI parts

The pcm179x family supports both SPI and I2C for configuration. This
patch splits the driver into core and SPI parts, in preparation for
I2C support.

Signed-off-by: Jacob Siverskog <jacob@teenage.engineering>
Reviewed-by: Johan Hovold <johan@kernel.org>
Acked-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Mark Brown <broonie@kernel.org>