Arm backend: Add docstrings for operator_support/ethos_u55_support.py

[Sebastian-Larsson]

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14774

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❌ 1 New Failure, 1 Unrelated Failure

As of commit a206c43 with merge base 3b16bc1 ():

NEW FAILURE - The following job has failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 5fb89430cf351e2a485d8f11ac24742b21984e58ed85a21839a8fd3654d7ed25 /exec failed with exit code 1

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 74

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[Sebastian-Larsson]

Unrelated CI failure