thermal: intel: hfi: Refactor enabling code into helper functions

In preparation for the addition of a suspend notifier, wrap the logic to
enable HFI and program its memory buffer into helper functions. Both the
CPU hotplug callback and the suspend notifier will use them.

This refactoring does not introduce functional changes.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>