cpufreq: amd-pstate: Add fast switch function for AMD P-State

Introduce the fast switch function for AMD P-State on the AMD processors
which support the full MSR register control. It's able to decrease the
latency on interrupt context.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>