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

Documentation/driver-api/conf.py

+# -*- 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'),
+]

Documentation/driver-api/index.rst

@@ -25,3 +25,11 @@ available subsections can be seen below.
    hsi
    miscellaneous
    vme
+
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`