variants(C0): add generic C011D6, C011F(4-6)(P-U) and C031F(4-6)P

[fpistm]

Base for #1995.

/CC @honnet

[honnet]

Very cool!
Now I understand better what was wrong in my original pull request, hopefully it will help me improve my future PRs, thanks a lot!

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PS: There's a last question if I may:

I had to remove the "WEAK" attribute for SystemClock_Config(), otherwise some of these parameters* were overwritten.

=> Could it be because it was (incorrectly) in variant_generic.cpp instead of generic_clock.c?

*I think it was the Clock source or pre-scaler.

[fpistm]

The function have to be WEAK to be able to overwrite it at sketch level that's all.

[honnet]

OK! Thanks!