dtoc: Add some debugging when copying nodes

Show the operations being performed, when debugging is enabled.

Convert a mistaken 'print' in test_copy_subnodes_from_phandles() while we
are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/test_fdt.py b/tools/dtoc/test_fdt.py
index 84dcd8b..f77e48b 100755
--- a/tools/dtoc/test_fdt.py
+++ b/tools/dtoc/test_fdt.py
@@ -32,6 +32,7 @@
 import libfdt
 from u_boot_pylib import test_util
 from u_boot_pylib import tools
+from u_boot_pylib import tout
 
 #pylint: disable=protected-access
 
@@ -414,7 +415,7 @@
 
         # Make sure that the phandle for 'over' is not copied
         over = dst.FindNode('over')
-        print('keys', over.props.keys())
+        tout.debug(f'keys: {over.props.keys()}')
         self.assertNotIn('phandle', over.props.keys())
 
         # Check the merged properties, first the base ones in '/dest'