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>