kernel/sched.c: refactor k_sleep() implementation for varied timescales

Current z_impl_k_sleep() does double duty, converting between units
specified by the API and ticks, as well as implementing the sleeping
mechanism itself. This patch separates the API from the mechanism,
so that sleeps need not be tied to millisecond timescales.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>