[stm32] start implementing timer_get_frequency

Get the timer clock frequency in order to more conveniently calculate the prescaler.

- untested and only valid for F1 and STM32F405xx/07xx and STM32F415xx/17xx
- does not handle/check TIMPRE in RCC_CFGR found in F42xxx and F43xxx

Ran out of time, so in case this sounds useful and someone wants to test it and/or extend it for more chips, here we go....