scripts/dts/gen_defines.py: Fix generation for multiple IO Channels

If there is more than one IO Channel than generate a define with a
trailing index for the IO Channel.  This matches what we do for GPIOs
and PWMs.

So something like:
  DT_FOOBAR_IO_CHANNELS_CONTROLLER_0
  DT_FOOBAR_IO_CHANNELS_CONTROLLER_1
  ...
  DT_FOOBAR_IO_CHANNELS_CONTROLLER_<N>

Fixes #18352

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>