[SYCL][DOC] Move proposed FPGA extensions

These two extension specifications are proposed changes to existing
extensions.  Move them to the "proposed" directory.  You can see the
proposed changes via:

```
$ git diff HEAD:sycl/doc/extensions/{supported,proposed}/SYCL_EXT_INTEL_DATAFLOW_PIPES.asciidoc
$ git diff HEAD:sycl/doc/extensions/{supported,proposed}/SYCL_EXT_INTEL_FPGA_LSU.md
```

Author: gmlueck