drivers: ethernet: ksz8081: support 25MHz RMII configuration

[DCoffey-CLN]

Configure KSZ8081 phy drivers to set strapping mode override for RMII mode without setting reference clock to 50MHz when "RMII 25MHz" operation is selected in device tree node.

[github-actions]

Hello @DCoffey-CLN, and thank you very much for your first pull request to the Zephyr project!
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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Hi @DCoffey-CLN!
Congratulations on getting your very first Zephyr pull request merged 🎉🥳. This is a fantastic achievement, and we're thrilled to have you as part of our community!

To celebrate this milestone and showcase your contribution, we'd love to award you the Zephyr Technical Contributor badge. If you're interested, please claim your badge by filling out this form: Claim Your Zephyr Badge.

Thank you for your valuable input, and we look forward to seeing more of your contributions in the future! 🪁