RemRam needs to redefine a particular timer to work correctly. The HAL did foresee this possibility and checks if the timer was already defined for either the stepper or temperature timer.

Prior to this commit the header file for the timers couldn't see those definitions in the pin file, but this commit will fix this.

Tested on an actual hardware to verify. (It wouldn't boot without it.)

Prescalers aren't calculated for the given target frequency.
Stepper timer should run at 2 Mhz and temperature timer at 72 Khz.

Always honor minimum period on stepper pulse generation, and fix timing calculations

Signed-off-by: etagle <ejtagle@hotmail.com>

Also simplify logic on all ARM-based interrupts. Now, it is REQUIRED to properly configure interrupt priority. USART should have highest priority, followed by Stepper, and then all others.

Followup to #9985

To reduce confusion over the current timer count vs. the compare (aka "top") value. Caution: this re-uses the function name, changing its meaning.