scripts: gen_handles: injected device handles

Update the device handle array for injected devices to output device
handles, not device ordinals. This requires that the extra ordinals
provided to `DEVICE_DT_INST` map to an existing device in order to
appear in the final handle array.

This is required for the injected handles to be useful, no meaningful
operations can be done on device ordinals.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>