ASoC: utils - Add support for a dummy codec driver.

This is useful to create dummy codec devices where we need to have some
DAI links without a real Codec. e.g. could be used to represent dumb FM,
MODEM, etc

This is also used by dynamic PCM for DAI links that have no codec.

Signed-off-by: Liam Girdwood <lrg@ti.com>
[Fixed the indentation -- broonie]
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>