The cs35l41 supports a low power DSP memory retention mode. Add support
for entering this mode when then device is not in use.

Co-authored-by: David Rhodes <david.rhodes@cirrus.com>
Signed-off-by: David Rhodes <david.rhodes@cirrus.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20220107160636.6555-3-ckeepax@opensource.cirrus.com


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