pylibfdt: Correct the FdtSw example

At present this example is incorrect since it is missing the call to
finish_reservemap() and does not add a root node. Fix these problems.

Signed-off-by: Simon Glass <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: David Gibson <[email protected]>
diff --git a/pylibfdt/libfdt.i b/pylibfdt/libfdt.i
index 643f95d..ecec4dd 100644
--- a/pylibfdt/libfdt.i
+++ b/pylibfdt/libfdt.i
@@ -727,8 +727,10 @@
 
     # First create the device tree with a node and property:
     sw = FdtSw()
-    with sw.add_node('node'):
-        sw.property_u32('reg', 2)
+    sw.finish_reservemap()
+    with sw.add_node(''):
+        with sw.add_node('node'):
+            sw.property_u32('reg', 2)
     fdt = sw.as_fdt()
 
     # Now we can use it as a real device tree