ASoC: mmp-sspa: Add support for soc-generic-dmaengine-pcm

ASoC: mmp-sspa: Add support for soc-generic-dmaengine-pcm

This makes the driver usable with the mmp_tdma drier via
soc-generic-dmaengine-pcm. This is conditionalized on DT node (support
for DT is added by a later patch).

A custom mmap callback that creates a NC mapping is used instead of the
default WC one, because with write-combining some bytes don't seem to
make it through for reasons unknown to me.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20200511210134.1224532-6-lkundrak@v3.sk
Signed-off-by: Mark Brown <broonie@kernel.org>