dmaengine: doc: ReSTize client API doc

This converts and moves client API file with some format
changes for RST style
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/dmaengine/index.rst

@@ -16,6 +16,17 @@ driver writers.
 
    provider
 
+DMAEngine client documentation
+------------------------------
+
+This book is a guide to device driver writers on how to use the Slave-DMA
+API of the DMAEngine. This is applicable only for slave DMA usage only.
+
+.. toctree::
+   :maxdepth: 1
+
+   client
+
 .. only::  subproject
 
    Indices