ASoC: dmaengine-pcm: Provide default config

This patch adds some default settings for the generic dmaengine PCM driver for
the case that no config has been supplied. The following defaults are used:
	* Use snd_dmaengine_pcm_prepare_slave_config for preparing the DMA slave
	  config.
	* 512kB for the prealloc buffer size. This value has been chosen based on
	  'feels about right' and is not backed up by any scientific facts. We
	  may need to come up with something smarter in the future but it should
	  work fine for now.

With this infrastructure in place we can finally write DAI drivers which are
independent of the DMA controller they are connected to. This is e.g. useful if
the DAI IP core is reused across different SoCs, but the SoCs uses different DMA
controllers.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>