ASoC: amd: ps: add comments for DMA register mapping

Add comments for DMA register mapping for both the SoundWire manager
instances.

Signed-off-by: Vijendar Mukunda <Vijendar.Mukunda@amd.com>
Link: https://lore.kernel.org/r/20230626105356.2580125-4-Vijendar.Mukunda@amd.com


Signed-off-by: Mark Brown <broonie@kernel.org>