doc-rst: fixed cleandoc target when used with O=dir

The cleandocs target won't work if I use a different output folder::

  $ make O=/tmp/kernel SPHINXDIRS="process" cleandocs
  make[1]: Entering directory '/tmp/kernel'
  make[3]: *** No rule to make target 'clean'.  Stop.
  ... Documentation/Makefile.sphinx:100: recipe for target 'cleandocs' failed

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