Add ADC triggers for L4xx devices

[dbristow-otc]

Sourced from RM0351 Rev 10 (STM32L47xxx, STM32L48xxx, STM32L49xxx and STM32L4Axxx
advanced Arm®-based 32-bit MCUs).
Tested on STM32L475VGT6 using the STM32L4x6 feature

[David-OConnor]

Good PR. Let's features gate it for "l4x6" only for now instead of all l4. I think the root cause of this is that this lib and the PAC (if stil true?) doesn't have a clear L4 variant breakdown. So, I'm not sure if this will work on all L4 variants.

[dbristow-otc]

Yes, it looks like the L4 has some oddities - I've never looked too deeply into the other variants, but it appears the L4x1, L4x2, and L4x3 only implement a subset of the triggers:

Should be safe to use the L4x5 and L4x6 features. The L4x5 PAC looks to still have a broken USB peripheral - I have a new SVD that supposedly corrects these issues, but it's easier to use the L4x6 features anyways.