ASoC: mxs: Use generic dmaengine PCM

Use the generic dmaengine PCM driver instead of a custom implementation.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>