doc: Add reinit-power-states to System PM docs

Add general purpose of and example use-case of the reinit-power-states
property added in this commit series.

Signed-off-by: Kenneth J. Miller <[email protected]>

Author: knthm

doc/services/pm/system.rst

@@ -93,3 +93,16 @@ applications to configure the policy manager to block system from transitioning
 into certain power states. This can be used by devices when executing tasks in
 background to prevent the system from going to a specific state where it would
 lose context.
+
+Some power states above S2RAM may also affect peripheral context; this can lead
+to a peripheral losing configuration when certain SoCs enters these specific
+low-power states.
+
+These exception states can be described in a peripheral's devicetree node with
+the ``reinit-power-states`` property:
+
+.. code-block:: devicetree
+
+   device-1 {
+      reinit-power-states = <&stop2>;
+   };