Documentation: dmaengine: Move the current doc to a folder of its own

Move the current client-side documentation to a subfolder to prepare the
introduction of a provider-side API documentation.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

diff --git a/Documentation/dmaengine.txt b/Documentation/dmaengine/client.txt
similarity index 100%
rename from Documentation/dmaengine.txt
rename to Documentation/dmaengine/client.txt