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.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>