cpufreq: amd-pstate: implement Pstate EPP support for the AMD processors

Add EPP driver support for AMD SoCs which support a dedicated MSR for
CPPC.  EPP is used by the DPM controller to configure the frequency that
a core operates at during short periods of activity.

The SoC EPP targets are configured on a scale from 0 to 255 where 0
represents maximum performance and 255 represents maximum efficiency.

The amd-pstate driver exports profile string names to userspace that are
tied to specific EPP values.

The balance_performance string (0x80) provides the best balance for
efficiency versus power on most systems, but users can choose other
strings to meet their needs as well.

$ cat /sys/devices/system/cpu/cpufreq/policy0/energy_performance_available_preferences
default performance balance_performance balance_power power

$ cat /sys/devices/system/cpu/cpufreq/policy0/energy_performance_preference
balance_performance

To enable the driver,it needs to add `amd_pstate=active` to kernel
command line and kernel will load the active mode epp driver

Acked-by: Huang Rui <[email protected]>
Reviewed-by: Mario Limonciello <[email protected]>
Reviewed-by: Wyes Karny <[email protected]>
Tested-by: Wyes Karny <[email protected]>
Signed-off-by: Perry Yuan <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: Perry Yuan