dt-bindings: ASoC: Add Spreadtrum multi-channel data transfer support

On Spreadtrum platform, the audio subsystem will use the multi-channel
data transfer controller to transfer sound stream between audio subsystem
and other AP/CP subsystem.

It can support 10 DAC channel and 10 ADC channel, and each channel has
512 bytes depth data fifo. Moreover each channel can be used DMA mode
or interrupt mode to transfer data.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>