doc: Move devicetree control doc to rST

Move this to rST format, largely unchanged to start with. Add an index
for this topic, as well as an empty intro.

Note this patch does not include updates! Is it just a conversion to the
new format. See the next patch.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchart <xypron.glpk@gmx.de>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 1203409..83c929b 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -11,6 +11,7 @@
 
    ci_testing
    commands
+   devicetree/index
    driver-model/index
    global_data
    logging