Support added for board NUCLEO-C031C6, cpu STM32C031C6, and CPU_FAM_STM32C0

[jparker324]

Support added for ST Micro's board NUCLEO-C031C6, it's cpu STM32C031C6, and the rest of the cpu family STM32C0XX (as CPU_FAM_STM32C0). This currently includes STM32C011 and STM32C031, but code for identification has also been added for the STM32C071 and STM32C091 lines due out later this year.

Code was adapted from CPU_FAM_STM32G0 where applicable.

The most important difference in the STM32C0 product line is the removal of the PLL. As coded the default clock source is HSI.

[jparker324]

Related issue #19210

[benpicco]

Great addition, thank you!
One minor thing though: can you please split the commit into two, so you have one commit for the CPU changes and one for adding the board?
Also please prefix your commits with cpu/stm32: for the CPU and boards/nucleo-c031c6 for the board.

Other than that, this looks really good already 😃

[benpicco]

Code looks good, just some small things:

[jparker324]

See PR with corrected commit history #20300

[benpicco]

You don't have to create a new PR, in the future when you edit your history with git rebase -i master you can just force push to your existing branch with git push -f