scripts: gen_defines: add a missing comment

Adding a comment at this point in the header file splits these macros
into their own section. They are presently grouped together with the
previous section, which is unrelated. This is confusing; fix it.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
diff --git a/scripts/dts/gen_defines.py b/scripts/dts/gen_defines.py
index d08f6e5..fdcc782 100755
--- a/scripts/dts/gen_defines.py
+++ b/scripts/dts/gen_defines.py
@@ -534,6 +534,8 @@
 def write_children(node):
     # Writes helper macros for dealing with node's children.
 
+    out_comment("Helper macros for child nodes of this node.")
+
     out_dt_define(f"{node.z_path_id}_FOREACH_CHILD(fn)",
             " ".join(f"fn(DT_{child.z_path_id})" for child in
                 node.children.values()))