stm32/cpu: Add functions for low power mode clock config

[bergzand]

Contribution description

This PR adds functions for managing the clock config in low power mode for the STM32 devices.

Testing procedure

I'm not really sure, it should at least compile for all stm32 based devices

Issues/PRs references

Required for #12556

[aabadie]

How about F0, F3 and L1 ?

[bergzand]

How about F0, F3 and L1 ?

As far as I could find, the F0, F1, F3 and L1 don't have these registers. No low power mode clock enable for them.

considering this case, do you think it makes sense to omit the function completely? The other solution is to add an empty function for those. I'm not really sure what the 'better' solution is in this case

[benpicco]

do you think it makes sense to omit the function completely? The other solution is to add an empty function for those. I'm not really sure what the 'better' solution is in this case

Depends, is it more convenient if you can just always can call that function even if it's a No-Op or would that code path not be taken on those devices because they need an entirely different driver?

[bergzand]

Depends, is it more convenient if you can just always can call that function even if it's a No-Op or would that code path not be taken on those devices because they need an entirely different driver?

I've been thinking about it a bit and I prefer to omit them for the cases where it would result in a No-op. The functions are internal and specific to a subset stm32 devices, they should only be used for periph drivers. In this case I prefer to have a hard fail on compilation compared to a silent "failure".

We can always add the No-op functions if it is more convenient on the periph side.

[benpicco]

Adds a new function, code looks clean and no side effects.

[bergzand]

Squashed!