drivers: pinctrl: stm32: Ignore NO_REMAP pins when handling AFIO remaps

[zyp]

Some peripherals (e.g. ethernet) have remaps only on some of the pins. Pins without remaps do not conflict and must be ignored to correctly process the remaps.

[github-actions]

Hello @zyp, and thank you very much for your first pull request to the Zephyr project!
Our Continuous Integration pipeline will execute a series of checks on your Pull Request commit messages and code, and you are expected to address any failures by updating the PR. Please take a look at our commit message guidelines to find out how to format your commit messages, and at our contribution workflow to understand how to update your Pull Request. If you haven't already, please make sure to review the project's Contributor Expectations and update (by amending and force-pushing the commits) your pull request if necessary.
If you are stuck or need help please join us on Discord and ask your question there. Additionally, you can escalate the review when applicable. 😊