kernel: Add new uniform time unit conversion API

Zephyr has always had an ad hoc collection of time unit macros and
conversion routines in a selection of different units, precisions,
rounding modes and naming conventions.

This adds a single optimized generator to produce any such conversion,
and enumerates it to produce a collection of 48 utilities in all
useful combinations as a single supported kernel API going forward.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>