ASoC: samsung: Specify DMA channel names through custom DMA config

This is a part of conversion of Samsung platforms to use the custom DMA
config for specifying DMA channel names, in addition to passing custom
DMA device for the secondary CPU DAI's "PCM" component for some variants
of the I2S controller.

We also don't set the SND_DMAENGINE_PCM_FLAG_CUSTOM_CHANNEL_NAME any more
as setting it wouldn't allow to specify DMA channels through the custom
DMA config.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>