ACPI / CPPC: Add a helper to get desired performance

This patch add a helper to get the value of desired performance
register.

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
[ rjw: More white space ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>