stm32* (others?): fixes required in lowerhalf timers

While working with the `arch_timer.c` driver, I noticed some issues in the implementation of lowerhalf timer (usually `_lowerhalf_tim.c`) of these architectures. If these are ever used with `arch_timer.c` it will fail. The changes are:
* allow settimeout to be called when the timer is started (there's generally no reason it cannot be done)
* implement `maxtimeout`

Architectures:
* stm32f0l0g0
* stm32
* stm32l4
* stm32f7
* nrf52 (in tim_lowerhalf, rtc_lowerhalf is OK)

These are all the ones that I found by looking for _tim_lowerhalf.c but if the naming is different in other cases there may be others affected. I suspect this is mostly because the implementation was copied across these different architectures.