dm: Provide a function to scan child FDT nodes

At present only root nodes in the device tree are scanned for devices.
But some devices can have children. For example a SPI bus may have
several children for each of its chip selects.

Add a function which scans subnodes and binds devices for each one. This
can be used for the root node scan also, so change it.

A device can call this function in its bind() or probe() methods to bind
its children.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/test.h b/include/dm/test.h
index 409f1a3..e8e1c0b 100644
--- a/include/dm/test.h
+++ b/include/dm/test.h
@@ -156,6 +156,15 @@
 			uint32_t base, struct dm_test_priv *priv);
 
 /**
+ * dm_check_devices() - check the devices respond to operations correctly
+ *
+ * @dms: Overall test state
+ * @num_devices: Number of test devices to check
+ * @return 0 if OK, -ve on error
+ */
+int dm_check_devices(struct dm_test_state *dms, int num_devices);
+
+/**
  * dm_test_main() - Run all the tests
  *
  * This runs all available driver model tests