ASoC: SOF: Add .prepare/.complete callbacks

Use SOF defined callbacks (snd_sof_prepare/snd_sof_complete)
in order to update internal SOF system suspend target.

Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Paul Olaru <paul.olaru@nxp.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20200924151518.15841-3-daniel.baluta@oss.nxp.com


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