docs: Add initial documentation for devfreq

.. SPDX-License-Identifier: GPL-2.0

========================

Device Frequency Scaling

========================

Introduction

------------

This framework provides a standard kernel interface for Dynamic Voltage and

Frequency Switching on arbitrary devices.

It exposes controls for adjusting frequency through sysfs files which are

similar to the cpufreq subsystem.

Devices for which current usage can be measured can have their frequency

automatically adjusted by governors.

API

---

Device drivers need to initialize a :c:type:`devfreq_profile` and call the

:c:func:`devfreq_add_device` function to create a :c:type:`devfreq` instance.

.. kernel-doc:: include/linux/devfreq.h

.. kernel-doc:: include/linux/devfreq-event.h

.. kernel-doc:: drivers/devfreq/devfreq.c

        :export:

.. kernel-doc:: drivers/devfreq/devfreq-event.c

        :export:

   ipmb

   i3c/index

   interconnect

   devfreq

   hsi

   edac

   scsi