ASoC: soc-generic-dmaengine-pcm: Add a DMA debugfs_prefix entry

After the conversion from platform to component the following warnings
are seen:

fsl-ssi-dai 2028000.ssi: ASoC: Failed to create component debugfs
directory
fsl-spdif-dai 2004000.spdif: ASoC: Failed to create component debugfs
directory

As explained by Mark Brown:

"It's a legit warning - we shouldn't really be creating two components
for the same device.  However this is a bit of a corner case as it's the
dmaengine platform driver which is kind of a virtual device, it's not
really the device that we use with DMA that's being represented but
rather the link between that and the DMA controller."

Disambiguate the DMA component name by adding a "dma" debugfs_prefix
entry.

Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>