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

Change all time conversions.  Yech.  New timer units in microseconds breaks all existing logic that used milliseconds in the conversions.  Something likely got broken doing this, probably because I confused a MSEC2TICK conversion with a TICK2MSEC conversion.  Also, the tickless OS no appears fully functional and passes the OS test on the simulator with no errors

Fix compler error in tickless code when round robin scheduling is enabled.  Other cosmetic fixes to comments