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 <sjg@chromium.org>
Message-Id: <20190703000815.102459-1-sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

pylibfdt/libfdt.i

@@ -727,8 +727,10 @@ class FdtSw(FdtRo):
 
     # 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