devicetree: stop using OrderedDict

Regular dicts are insertion-ordered since CPython 3.6 and Python 3.7.
Zephyr now requires Python 3.8, so it should be OK to replace
OrderedDict with regular dict now. This results in less typing and
more readable object representations.

A nitpicker could argue that this is a functional change, since if a
user is doing 'assert isinstance(node.props, OrderedDict)', that will
fail now, but:

 1. nobody is doing something like that in the zephyr tree
 2. that would be a silly thing to do
 3. we don't currently make any API stability guarantees
    for this module right now anyway

so it should be fine.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>