ASoC: SOF: imx8m: Implement DSP start

On i.MX8M DSP is controlled via a set of registers
from Audio MIX. This patches gets a reference (via regmap)
to Audio Mix registers and implements DSP start.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20211119094319.81674-5-daniel.baluta@oss.nxp.com


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