stm32: Enable hw pwm for pin PA3

[Jyrka98]

This adds support for using hardware pwm on pin PA3 on STM32H7 based boards. In my case, for FAN3 on a BTT Octopus Max EZ

[JamesH1978]

Thank you for submitting a PR, please be aware that you need to sign off to accept the developer certificate of origin, please see point 3 in https://github.com/Klipper3d/klipper/blob/master/docs/CONTRIBUTING.md#what-to-expect-in-a-review

Thanks James

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