Allow nonce in <link> tag

### Description

Hi, my team is currently implementing CSP for our web. The implementation including adding nonce to style, link, script, and iframe. But i stumbled on an error when adding nonce to AMP pages. Currently got `The attribute 'nonce' may not appear in tag 'link'` from AMP Validator. While `` and `<script>` are allowed to use `nonce` already ([#6992](https://github.com/ampproject/amphtml/issues/6992#issuecomment-274555542)).

Implementation:
```
<link rel="dns-prefetch" href="https://www.google-analytics.com" nonce="...">
```

Expectation:
AMP page is considered valid when it contains `<link nonce="...">`

The issue names no files or tests; begin by locating the AMP Validator rules for attributes and the existing nonce handling referenced from #6992. Confirm the change by validating an AMP page containing and ensuring the validator accepts it.

Written by the indexing model from the issue text.