ASoC: tegra: convert to standard DMA DT bindings

By passing no flags when calling snd_dmaengine_pcm_register() from
tegra_pcm.c, we end up using dma_request_slave_channel() rather than
dmaengine_pcm_compat_request_channel(), and hence rely on the standard
DMA DT bindings and stashing the DMA slave ID away during channel
allocation. This means there's no need to use a custom DT property to
store the slave ID. So, remove all the code that parsed it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Mark Brown <broonie@linaro.org>