fix: timers on STM32F411xE boards

[JeppeKlitgaard]

This PR fixes the TimerField for STM32F411xE boards such as the Nucleo F411RE board.

It preserves compatibility with the existing supported board (STM32F429xx) and introduces an error for boards that haven't been explicitly defined as working. Adding new boards is easy - simply look up the timers in the documentation and add the appropriate define clause.

Note that I have not run tests on STM32F429xx's, as I don't have access to any. @ramboerik if you still have the hardware and feel like it, it would be fantastic if you could confirm that this works for your setup.