dm: Rename device_get_by_driver_info_idx()

This function finds a device by its driver_info index. With
of-platdata-inst we do not use driver_info, but instead instantiate
udevice records at build-time.

However the semantics of using the function are the same in each case:
the caller provides an index and gets back a device.

So rename the function to device_get_by_ofplat_idx(), so that it can be
used for both situations. The caller does not really need to worry about
the details.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>