dm: core: Add functions for iterating through device children

Buses need to iterate through their children in some situations. Add a few
functions to make this easy.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
diff --git a/doc/driver-model/README.txt b/doc/driver-model/README.txt
index f9b68be..c435cdc 100644
--- a/doc/driver-model/README.txt
+++ b/doc/driver-model/README.txt
@@ -95,7 +95,7 @@
 You should see something like this:
 
     <...U-Boot banner...>
-    Running 21 driver model tests
+    Running 22 driver model tests
     Test: dm_test_autobind
     Test: dm_test_autoprobe
     Test: dm_test_bus_children
@@ -103,6 +103,7 @@
     Device 'c-test@0': seq 0 is in use by 'a-test'
     Device 'c-test@1': seq 1 is in use by 'd-test'
     Test: dm_test_bus_children_funcs
+    Test: dm_test_bus_children_iterators
     Test: dm_test_bus_parent_data
     Test: dm_test_bus_parent_ops
     Test: dm_test_children