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>

Documentation/driver-api/conf.py

@@ -0,0 +1,10 @@
+# -*- coding: utf-8; mode: python -*-
+
+project = "The Linux driver implementer's API guide"
+
+tags.add("subproject")
+
+latex_documents = [
+    ('index', 'driver-api.tex', project,
+     'The kernel development community', 'manual'),
+]