ASoC: SOF: Add support ctx_save with IPC4

Merge series from Peter Ujfalusi <peter.ujfalusi@linux.intel.com>:

The context save functionality with IPC4 is triggered by sending a message to
the firmware about the pending power down of the primary core by the host.

In order to have this functionality implemented in a clean way we need to
introduce a new IPC level PM ops for core state management and use that instead
of open coding IPC messages here and there.

The first patch updates the ctx store/ctx_restore documentation to clarify that
they are optional.