Tickless Stuff:  Back out the risky timer operations when the ready-to-run list is modified.  That is unsafe.  An ugly workaround is just to keep an interval timer going all of the time with a minimum duration equal to the timeslice interval.

Timers: In Tickless mode, need to stop the interval timer before inserted a new delay into the timer list.  Otherwise, the time is incorrect on the first entry of the list

SAMA5 free-running timer: Add support for a free-running timer wrapper around the low-level timer/counter logic.

SAMA5 timer/counter: Add support for a one-shot timer wrapper around the low-level timer/counter logic.  This also involved several changes that rippled into the ADC driver (untested).