cpufreq: intel_pstate: Replace bxt_funcs with core_funcs

Since core_funcs and bxt_funcs have same set of callbacks, replace
bxt_funcs with core_funcs.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>