onnxruntime

update onnxruntime_perftest's README.md as vitisai is supported on v1.15.1 (#16827)

### Description
<!-- Describe your changes. -->
Updating README.md to add vitisai for onnxruntime_perftest


### Motivation and Context
<!-- - Why is this change required? What problem does it solve? -->
The perftest tool does support vitisai whereas the README.md does not
list it. This creates some confusions internally about if vitisai is
supported. See https://github.com/microsoft/onnxruntime/pull/15673 for
context.