Allow step rates less than 32 steps/sec (fix issue #986)

Change block initialization in planner.cpp to allow motion rates less
than 32 steps/sec.

Old behavior: planner.cpp set stepper interrupt rate to the highest step
rate among steppers. In case that rate turned out to be too slow,
stepper.cpp increased stepper interrupt rate to the minimum supported
rate, that increased motion rate.

New behavior: introduce a new macros STEPPER_MIN_TICK_RATE in stepper.h.
In case nominal_rate is too low, increase both step_event_count and
nominal_rate proportionally.

This fixes issue https://github.com/ErikZalm/Marlin/issues/986