TFLu: Update Ethos-U kernel

[mansnils]

Update with new TfLiteEvalTensor API.
Also resolve error, variable length array by using scratch buffer
request/get.

FIXED=166478549

[gbaned]

Thanks for contributing to TensorFlow Lite Micro.

To keep this process moving along, we'd like to make sure that you have completed the items on this list:
Read the contributing guidelines for TensorFlow Lite Micro
Created a TF Lite Micro Github issue
Linked to the issue from the PR description

We would like to have a discussion on the Github issue first to determine the best path forward, and then proceed to the PR review.

[advaitjain]

Thanks @mansnils for breaking up the work into small PRs.

I have edited the PR description (by adding FIXED=166478549) as a test to see if the PR description gets pulled into our internal systems with the appropriate formatting to close internal bugs (similar to how we can close issues on github).

This is just an FYI for you, no action needed from your side.