patman: Use items() to iterate over dictionaries

In python 3.x the iteritems() method has been removed from dictionaries,
and the items() method does effectively the same thing. On python 2.x
using items() is a little less efficient since it involves copying data,
but as speed isn't a concern in this code switch to using items() anyway
for simplicity.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Simon Glass <sjg@chromium.org>