[TOSA] Update tfl.mean legalization to match the new reference kernel behaviour
@Tessil FYI Hi @jpienaar and @rsuderman, please could you help to review when you have time.
Thank you! Hi @jpienaar / @rsuderman Can you please review this PR ? Thank you! Force-pushed the PR feature branch after resolving merge conflicts caused by rebasing. > Could you add a test for these changes. Other than that the PR looks good.

I have add new mlir test for quantized reduce_mean.
This PR updates the quantized REDUCE_MEAN kernel to only use integers. This matches the TFLite reference model in a bit-exact way.
(The merged PR:https://github.com/tensorflow/tensorflow/pull/52014 updates the quantized REDUCE_MEAN kernel to only use integers)
