docs: Add initial documentation for devfreq

The devfreq subsystem has plenty of kernel-doc comments but they're not
currently included in sphinx documentation.

Add a minimal devfreq.rst file which mostly just includes kernel-doc
comments from devfreq source. This also exposes a number of kernel-doc
warnings on `make htmldocs`
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Link: https://lore.kernel.org/r/e32fa9de8a60060a6ee5fc42f163111034f9a550.1574181341.git.leonard.crestez@nxp.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/devfreq.rst

+.. 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:

Documentation/driver-api/index.rst

@@ -39,6 +39,7 @@ available subsections can be seen below.
    ipmb
    i3c/index
    interconnect
+   devfreq
    hsi
    edac
    scsi