doc: release: 2.7: add release notes for PM

    together have allowed simplifying multiple device power management callback

    implementations.

  * Introduced a new API to allow devices capable of wake up the system

    register themselves was wake up sources. This permits applications to

    select the most appropriate way to wake up the system when it is

    suspended. Devices marked as wake up source are not suspended by the kernel

    when the system is idle. It is possible to declare a device wake up capable

    direct in devicetree like this example::

        &gpio0 {

                compatible = "zephyr,gpio-emul";

                gpio-controller;

                wakeup-source;

        };

    * Removed  ``PM_DEVICE_STATE_FORCE_SUSPEND`` device power state.because it

      is an action and not a state.

    * Removed ``PM_DEVICE_STATE_RESUMING`` and ``PM_DEVICE_STATE_SUSPENDING``.

      They were transitional states and only used in device runtime. Now the

      subsystem is using device flag to keep track of a transition.

    * Implement constraint API as weak symbols so applications or platform

      can override them. Platforms can have their own way to

      set/release constraints in their drivers that are not part of

      Zephyr code base.

* Logging

* MODBUS