Enable other STM32 architectures.

[honnet]

Hi @arpruss!

Following up on the issue #9, here is a proposition that might be good for other ADCTouchSensor users who would want to try the STM32duino (it's supported by ST so they have a lot of new architectures).

There is no default option for ADCTOUCH_DIVIDER, so another even more inclusive option could be to use #else instead of my proposed #elif defined(ARDUINO_ARCH_STM32) (see here).

Let me know if you prefer this 2nd option?

...or if there is anything I could do to make this pull request better?

Thanks again for this great lib! Cedric :)

---

PS: It seems that Roger Clark suggests to use the stm32duino core now: https://github.com/rogerclarkmelbourne/Arduino_STM32#background--support