dtlib: implement copy.deepcopy() for DT

The standard library copy module allows you to implement shallow and
deep copies of objects. See its documentation for more details on
these terms.

Implementing copy.deepcopy() support for DT objects will allow us to
"clone" devicetree objects in other classes. This in turn will enable
new features, such as native system devicetree support, within the
python-devicetree.

It is also a pure feature extension which can't harm anything and is
therefore safe to merge now, even if system devicetree is never
adopted in Zephyr.

Note that we are making use of the move from OrderedDict to regular
dict to make this implementation more convenient.

See https://github.com/devicetree-org/lopper/

 for more information on
system devicetree. We want to add system devicetree support to dtlib
because it seems to be a useful way to model modern, heterogeneous
SoCs than traditional devicetree, which can really only model a single
CPU "cluster" within such an SoC.

In order to create 'regular' devicetrees from a system devicetree, we
will want a programming interface that does the following:

   1. parse the system devicetree
   2. receive the desired transformations on it
   3. perform the desired transformations to make
      a 'regular' devicetree

Step 3 can be done as a destructive modification on an object-oriented
representation of a system devicetree, and that's the approach we will
take in python-devicetree. It will therefore be convenient to have an
efficient deepcopy implementation to be able to preserve the original
system devicetree and the derived regular devicetree in memory in the
same python process.

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