Corstone-320 support (#5628)

Summary:
Adds support for compiling for Corstone-320 FVP containing Ethos-U85.

Follow up PR will add downloading and running of the FVP once it's available.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5628

Reviewed By: digantdesai

Differential Revision: D63637308

Pulled By: mergennachin

fbshipit-source-id: 2d87575a46e0f6be0ccdafe4969215dab566b5ff