Support wm_adsp hibernation for runtime suspend

Merge series from Stefan Binding <sbinding@opensource.cirrus.com>:

When the CS35L41 and CS35L45 drivers suspend, they are put into
hibernation, and the regmap goes into cache_only, but the firmware is
still running, and wm_adsp is not stopped. If userspace attempts to
read a firmware control, it will perform a regmap_raw_read() and this
will produce an error in the kernel log.

To prevent these spurious errors, add an apis into cs_dsp and wm_adsp
to allow wm_adsp to hibernate. In this hibernation mode, reads or
writes to the dsp controls would be rejected with -EPERM rather than
-EBUSY, and no error will be printed to the kernel log.