pm: policy: allow using latency API in standalone mode

This option allows using the pm_policy_latency* APIs to gather latency
requirements on systems that do not support PM (e.g. systems whithout
CPU idle states). Because the API has a subscription mechanism, it can
be useful to perform system-level adjustments based on latency
requirements gathered from multiple system components.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>