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>

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;