docs: pm: Add missing device states

Adding documentation about two new device states, SUSPENDING and
RESUMING.

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

Author: Flavio Ceolin

doc/reference/power_management/index.rst

@@ -225,6 +225,16 @@ The four device power states:
    Most device context is lost by the hardware. Device drivers must save and
    restore or reinitialize any context lost by the hardware.
 
+:code:`PM_DEVICE_STATE_SUSPENDING`
+
+   Device is currently transitioning from :c:macro:`PM_DEVICE_STATE_ACTIVE` to
+   :c:macro:`PM_DEVICE_STATE_SUSPEND`.
+
+:code:`PM_DEVICE_STATE_RESUMING`
+
+   Device is currently transitioning from :c:macro:`PM_DEVICE_STATE_SUSPEND` to
+   :c:macro:`PM_DEVICE_STATE_ACTIVE`.
+
 :code:`PM_DEVICE_STATE_OFF`
 
    Power has been fully removed from the device. The device context is lost