drivers: systick: Fix Cortex-M systick jumping forward and back again

The existing implementation did not properly
handle when `SysTick->VAL` is zero.

This caused three subtle edge cases:
* val1=0,COUNTFLAG=0,val2=0
  This should result in no cycles elapsed,
  however `(last_load - val2) = last_load`.
  So an extra `last_load` cycles was returned.
* val1=0,COUNTFLAG=0,val2=(last_load-1)
  This should result in 1 cycle elapsed,
  however `val1 < val2` so an extra `last_load`
  cycles was returned.
* val1=[2,1,0],COUNTFLAG=1,val2=0
  This should result in `last_load` cycles elapsed.
  However, `last_load * 2` cycles was returned.

To fix the calculation, val1 and val2 are first
wrapped/realigned from [0:last_load-1] to [1:last_load].

Tidy comments to better reflect the SysTick
behaviour and link reference manuals.

Signed-off-by: Grant Ramsay <gramsay@enphaseenergy.com>