arch/xtensa: Add CCOUNT-based timing API

Expose the Xtenesa CCOUNT timing register (the lowest level CPU cycle
counter) using the arch_timing_*() API.

This is the simplest possible way to get this working.  Future work
might focus on moving the rate configuration into devicetree in a
standard way, integrating with the platform clock driver on intel_adsp
such that the reported cycle rate tracks runtime changes (though IIRC
this is not a SOF requirement), and adding better test coverage to the
timing layer, which right now isn't exercised anywhere but in
benchmarks.

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