ASoC: SOF: add a power_down_notify method

This patch adds a snd_sof_dsp_power_down_notify() method which aims to
prepare the DSP for the upcoming power down.
This new function make use of SOF_IPC_PM_CTX_SAVE message.

Signed-off-by: Marcin Rajwa <marcin.rajwa@linux.intel.com>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20200515135958.17511-3-kai.vehmanen@linux.intel.com


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