doc-rst: make driver-api folder buildable stand-alone

Add minimal conf.py makes the driver-api folder buildable
stand-alone. To build only this folder run::

  make SPHINXDIRS=driver-api htmldocs
  make SPHINXDIRS=driver-api pdfdocs

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>