scripts: dtlib/edtlib: Make output consistent on Python 3.5-

Python 3.5 and earlier do not preserve dictionary insertion order when
iterating over dictionaries, and do not give the same order between
runs. This broke the dtlib and edtlib test suites and made the output
jump around randomly between runs. It also made device INST_<n> numbers
non-deterministic, which broke some code on Python 3.5 (though
hardcoding device instance numbers in the code might be a bit shaky).

Fix it by using collections.OrderedDict instead of plain dict wherever
order matters. This makes the output identical on all supported Python
versions. It also allows testdtlib.py and testedtlib.py to run in CI,
which uses Python 3.5.

Fixes: #20571

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>