pm: device: runtime: refactor API

This patch refactors the runtime API to make it more clear and simple.

Relevant changes:

- API uses the action callback in a direct manner, avoiding unnecessary
  overhead.
- API documentation has been improved to include detailed return error
  codes.
- pm_runtime_disable() is now synchronous (to simplify possible error
  paths) and returns error in case it fails. It is also safe to disable
  in pre-kernel now.
- pm_runtime_put(_async)() will return -EALREADY if called with usage
  count at zero (result of an unbalanced get/put call sequence)
- A transitional state has been added back. This makes code more
  readable, and avoids using atomics (not required).

TODO:

- Solve in a better manner the asynchronous suspend error path (now
  "solved" using asserts).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
4 files changed