[Ethos-U][TIR] Handle DeclBuffer in Ethos-U inputs

[Lunderberg]

This is a subset of changes, being split out from #14778 into independent portions. For the current TIR that is present at the end of lowering, no behavior changes are expected, as no DeclBuffer nodes are present.

[tvm-bot]

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• cc @Hzfengsy, @junrushao, @quic-sanirudh, @shingjan _{See #10317 for details}

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