[CMSIS-NN] Support for Softmax Int16 operator

[Nikeran]

• Support for int16 Softmax in CMSIS-NN
• Supporting integration test

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• cc @areusch, @ashutosh-arm _{See #10317 for details}

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[asparkhi]

cc @lhutton1

[lhutton1]

Thanks @Nikeran this looks great, thanks for the hard work on it! I had mostly stylistic changes and one comment that might be a bit more involved - happy to go through it separately

[Aleksei-grovety]

Thanks @Nikeran! Looks good just a couple questions.

[Nikeran]

@lhutton1 would you take a look at the update to see if you find anything that needs changing?

[lhutton1]

LGTM!

[lhutton1]

Thanks @Nikeran @Aleksei-grovety @ashutosh-arm!