[Bridge] [DoctrineExtension] Allow cache drivers that are not an EM's child

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes|]
| Fixed tickets | []
| License       | MIT
| Doc PR        | [https://github.com/doctrine/doctrine2/pull/808, https://github.com/doctrine/DoctrineBundle/pull/224]