dtoc: add missing code comments

Add missing information about internal class members in order to make
the code easier to follow.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/dtb_platdata.py b/tools/dtoc/dtb_platdata.py
index ecfe062..bc0de42 100644
--- a/tools/dtoc/dtb_platdata.py
+++ b/tools/dtoc/dtb_platdata.py
@@ -140,6 +140,9 @@
         _include_disabled: true to include nodes marked status = "disabled"
         _outfile: The current output file (sys.stdout or a real file)
         _lines: Stashed list of output lines for outputting in the future
+        _aliases: Dict that hold aliases for compatible strings
+            key: First compatible string declared in a node
+            value: List of additional compatible strings declared in a node
     """
     def __init__(self, dtb_fname, include_disabled):
         self._fdt = None