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

Change CONFIG_MSEC_PER_TICK to CONFIG_USEC_PER_TICK.  This gives more options for system timers in general, but more importantly, let's us realize higher resolution for the case of CONFIG_SCHED_TICKLESS=y -- of course, at the risk of some new interger overvflow problems