[STM32] Handle STM32H7 "_C" pins

[JuliDi]

As discussed in the chat, the split-pxy features are introduced for use with embassy-rs/stm32-data#254

Ideas for improvement:

• Verify this actually works with the split functions enabled (the traits are there, but I have no device that actually exposes the PC2/3 pins)
• Add a testcase / CI step for compilation with these flags. Any suggestions where that should go? Or maybe a HIL test on an H7 board?

[Dirbaio]

Add a testcase / CI step for compilation with these flags. Any suggestions where that should go?

yes, would be good to add a few test cases of building embassy-stm32 with/without the split-xx features to ci.sh.

Or maybe a HIL test on an H7 board?

perhaps later, since we don't even have ADC hil tests for now...

[JuliDi]

Won't be able to work on this for the next 2 weeks or so, but I'm happy to receive feedback in the meantime and make a proper implementation later on.

[JuliDi]

closed in favor of #1988