ASoC: wm_adsp: Expose the DSP power down actions as wm_adsp_power_down()

ASoC: wm_adsp: Expose the DSP power down actions as wm_adsp_power_down()

To support self-booting DSPs that operate outside of a conventional DAPM
event life cycle expose a companion function to wm_adsp_power_up() so
that the correct state of the DSP firmware and controls can be recorded.

Signed-off-by: Simon Trimmer <simont@opensource.cirrus.com>
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20230808164702.21272-5-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>