feat(material): simple strings as textPreffix/textSuffix for form-fields

[RomainPrevot]

Update form field wrapper to handle simple text in textPreffix/textSuffix

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Small feature for the form field wrapper

What is the current behavior? (You can also link to an open issue here)
The textPreffix and textSuffix options only accept templateRef, but the hint accept both strings and templateRef

What is the new behavior (if this is a feature change)?
The textPreffix and textSuffix options now handle both strings and templateRef (like the hint)

Please check if the PR fulfills these requirements

• The commit messages follow our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• A unit test has been written for this change.
• Running npm run build produced a successful build. (Unit testing can be done by running npm test;)
• My code has been linted. (npm run lint to do this. npm run build will fail if there are files not linted.)

Please provide a screenshot of this feature before and after your code changes, if applicable.

Other information:
Didn't found any unit test for this part