ASoC: davinci-mcasp: Extract DMA channels directly from DT

Extract DMA channels directly from DT as they can not be found from
platform resources anymore. This is a work-around until davinci audio
driver is updated to use dmaengine.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>