Change `yxi` backend to look for `ref` memories in addition to `@external` memories

[nathanielnrn]

The AXI generator currently expects programs to use ref to denote memories expected to interface with the XRT shell via AXI. the yxi backend currently only looks for @external memories.

A pass is coming that will change @external to ref #1918, but the yxi backend should support both cases to provide compatibility with programs that use @external.

This would probably have to go along with #1933

[nathanielnrn]

Unfortunately this is not working as intended, see #1993