doc: pm: Update system device pm

Add new information about new property to
enable/disable device power management per
power state.

Signed-off-by: Flavio Ceolin <[email protected]>

doc/services/pm/device.rst

@@ -44,12 +44,39 @@ When using this type, device power management is mostly done inside
 :c:func:`pm_system_suspend()` along with entering a CPU or SOC power state.
 
 If a decision to enter a CPU lower power state is made, the power management
-subsystem will suspend devices before changing state. The subsystem takes care
-of suspending devices following their initialization order, ensuring that
+subsystem will check if the selected low power state triggers device power
+management and then suspend devices before changing state. The subsystem takes
+care of suspending devices following their initialization order, ensuring that
 possible dependencies between them are satisfied. As soon as the CPU wakes up
 from a sleep state, devices are resumed in the opposite order that they were
 suspended.
 
+The decision about suspending devices in a low power state is done checking
+if the power state has the property ``zephyr,pm-device-enabled``. Here is an
+example of a target with two low power states with only triggering device
+power management:
+
+.. code-block:: devicetree
+
+   /* Node in a DTS file */
+   cpus {
+        power-states {
+                state0: state0 {
+                        compatible = "zephyr,power-state";
+                        power-state-name = "standby";
+                        min-residency-us = <5000>;
+                        exit-latency-us = <240>;
+                };
+                state1: state1 {
+                        compatible = "zephyr,power-state";
+                        power-state-name = "suspend-to-ram";
+                        zephyr,pm-device-enabled;
+                        min-residency-us = <8000>;
+                        exit-latency-us = <360>;
+                };
+        };
+   };
+
 .. note::
 
    When using :ref:`pm-system`, device transitions can be run from the idle thread.