cpufreq: intel_pstate: Replace BYT with ATOM

Rename symbol and function names starting with "BYT" or "byt" to
start with "ATOM" or "atom", respectively, so as to make it clear
that they may apply to Atom in general and not just to Baytrail
(the goal is to support several Atoms architectures eventually).

This should not lead to any functional changes.

Signed-off-by: Philippe Longepe <philippe.longepe@linux.intel.com>
Signed-off-by: Stephane Gasparini <stephane.gasparini@linux.intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
[ rjw : Changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>