dm: test: Add test case for dev_phys_to_bus()/dev_bus_to_phys()

By reusing DT nodes already available in sandbox's test DT introduce a
test to validate dev_phys_to_bus()/dev_bus_to_phys().

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>