pylibfdt: Allow reading integer values from properties

Extend the Properties class with some functions to read a single integer
property. Add a new getprop_obj() function to return a Property object
instead of the raw data.

This suggested approach can be extended to handle other types, as well as
arrays.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>