dtoc: Use items() to iterate over dictionaries in python 3.x

dtoc: Use items() to iterate over dictionaries in python 3.x

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 the affected code switch to using
items() anyway for simplicity.

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