pm: policy: move constraints to policy API

The pm_constraint_* APIs were effectively used by the policy manager
only. This patch renames the API to the policy namespace and makes its
naming more explicit:

- pm_constraint_set -> pm_policy_state_lock_get()
- pm_constraint_release -> pm_policy_state_lock_put()
- pm_constraint_get -> pm_policy_state_lock_is_active()

The reason for these changes is that constraints can be of many types:
allow/disallow states, impose latency requirements, etc. The new naming
also makes explicit that the API calls will influence the PM policy
behavior.

All drivers and documentation have been updated accordingly.

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

drivers/serial/uart_stm32.h

@@ -78,7 +78,7 @@ struct uart_stm32_data {
 #ifdef CONFIG_PM
 	bool tx_poll_stream_on;
 	bool tx_int_stream_on;
-	bool pm_constraint_on;
+	bool pm_policy_state_on;
 #endif
 };