Portable selected op workaround for arm backend (#1122)

Summary:
Arm uses portable yaml, and it only needs softmax. However, now we disabled root op list, and can only support yaml, so this breaks arm build.

We temporarily support yaml and ops list while we make a selective build rule for arm.

Pull Request resolved: https://github.com/pytorch/executorch/pull/1122

Reviewed By: larryliu0820

Differential Revision: D50936359

Pulled By: kirklandsign

fbshipit-source-id: b220d3526f2a40769ebd06b492a3563da299920d