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).

SAMA5 T/C: Can now handle non-constant BOARD_MCK_FREQUENCY. Also now supports methods to attach user interrupt handlers

Remove os_internal.h it has been replace by several new header files under sched/.  There have been some sneak inclusion paths via os_internal.h, so expect a few compilation errors for some architectures