kernel/timeout: Add k_uptime_ticks() API

Add a call to get the system tick count as an official API (and
redefine the existing millisecond API in terms of it).  Sophisticated
applications need to be able to count ticks directly, and the newer
timeout API supports that.  Uptime should too, for symmetry.

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