dmaengine: doc: ReSTize provider doc

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

Documentation/driver-api/dmaengine/index.rst

@@ -5,6 +5,17 @@ DMAEngine documentation
 DMAEngine documentation provides documents for various aspects of DMAEngine
 framework.
 
+DMAEngine documentation
+-----------------------
+
+This book helps with DMAengine internal APIs and guide for DMAEngine device
+driver writers.
+
+.. toctree::
+   :maxdepth: 1
+
+   provider
+
 .. only::  subproject
 
    Indices