devicetree: add mypy-based type checking

Extend the steps taken in tox.ini by type checking the 'devicetree'
package. This will make it easier for callers to type-check code that
uses the low level DT module's public APIs, since they can rely on
the type checker a bit more.

It will also help avoid bugs by adding some type checking for future
changes to this module.

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