dm: core: Set device tree node for root device

The root device corresponds to the root device tree node, so set this up.
Also add a few notes to the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/driver-model/README.txt

@@ -363,6 +363,10 @@ can leave out platdata_auto_alloc_size. In this case you can use malloc
 in your ofdata_to_platdata (or probe) method to allocate the required memory,
 and you should free it in the remove method.
 
+The driver model tree is intended to mirror that of the device tree. The
+root driver is at device tree offset 0 (the root node, '/'), and its
+children are the children of the root node.
+
 
 Declaring Uclasses
 ------------------

drivers/core/root.c

@@ -49,6 +49,9 @@ int dm_init(void)
 	ret = device_bind_by_name(NULL, false, &root_info, &DM_ROOT_NON_CONST);
 	if (ret)
 		return ret;
+#ifdef CONFIG_OF_CONTROL
+	DM_ROOT_NON_CONST->of_offset = 0;
+#endif
 	ret = device_probe(DM_ROOT_NON_CONST);
 	if (ret)
 		return ret;