Cadence - Move primary code to backends folder (#3353)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3353

See design and discussion in https://docs.google.com/document/d/1HPDTbN07WXB9PCdezmvSs_0Yy89D57F1pHHAF9UAgGg/edit#heading=h.828btb3wp67h.

Previous folder structure:
```
executorch
└── examples
    ├── aot
    ├── kernels
    ├── ops
    ├── tests
    ├── third-party/hifi4-nnlib
    └── utils
```

New folder structure:
```
executorch
├── backends
│   └── cadence
│       ├── aot
│       ├── ops_registration
│       ├── tests
│       ├── utils
│       ├── hifi
│       │   ├── kernels
│       │   ├── operators
│       │   └── third-party
│       │       └── nnlib
│       └── [other cadence DSP families]
│           ├── kernels
│           ├── operators
│           └── third-party
│               └── [any required lib]
└── examples
    └── cadence
        ├── models
        └── operators
```

Reviewed By: tarun292, cccclai

Differential Revision: D56577399

fbshipit-source-id: a19d7d689b286c0da2ef533a17e5e66ee1eb8a26