ASoC: cs35l56: Add system suspend handling

Merge series from Richard Fitzgerald <rf@opensource.cirrus.com>:

This set of patches adds handling for system suspend.
Patches 1..4 make some code changes that simplify the
suspend implementation, mainly to avoid race conditions.

There are two seperate aspects to suspend, and these have
been done as two patches:
- the main suspend-resume handling,
- re-loading the firmware if necessary after resume.