Some core peripherals get an API

[bergus]

I think #74 was supposed to fix that. However, in the STM32l4x2.svd that I use, there are peripheral definitions for STK (the systick, SYST) as well as for FPU_CPACR and SCB_ACTRL. I do not know how common such discrepancies are, but what's the best way to fix them?

My guess:

• For STK, the peripheral should be renamed to the proper SYST, like in @adamgreig's stm32-rs project
• For those peripherals with multiple separate register blocks, detect whether one of the known core peripherals is a prefix of the peripheral name