Update of Documentation/dmaengine/00-INDEX

- client.txt was moved by f36d2e67 (dmaengine: Move the current doc to a
  folder of its own)
- dmatmest.txt was moved by 935cdb56 (dmanegine: move dmatest.txt to
  dmaengine folder)
- provider.txt was added by c4d2ae96 (Documentation: dmaengine: Add a
  documentation for the dma controller API).

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: dmaengine@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Signed-off-by: Henrik Austad <henrik@austad.us>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>