node: support for useful properties accessor (#27)

Add support for parent (node) access from a given node. If node is root,
returning oneself.
Add support for node address or identifier (id field) when using such a
definition:
```dts
 label: name@id {
 };
 ```
 id can be an address or an identifier.

Author: pthierry-ledger

src/dts_utils/node.py

@@ -32,6 +32,14 @@ def label(self):
             return self._node.labels[0]
         return ""
 
+    @property
+    def parent(self):
+        return self._node.parent
+
+    @property
+    def unit_addr(self):
+        return self._node.unit_addr
+
     def __getattr__(self, __name: str) -> Any:
         from .property import Property