cpufreq: User/admin documentation update and consolidation

The user/admin documentation of cpufreq is badly outdated.  It
conains stale and/or inaccurate information along with things
that are not particularly useful.  Also, some of the important
pieces are missing from it.

For this reason, add a new user/admin document for cpufreq
containing current information to admin-guide and drop the old
outdated .txt documents it is replacing.

Since there will be more PM documents in admin-guide going forward,
create a separate directory for them and put the cpufreq document
in there right away.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/pm/index.rst

@@ -0,0 +1,15 @@
+================
+Power Management
+================
+
+.. toctree::
+   :maxdepth: 2
+
+   cpufreq
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`