power: add p-state driver compatibility

power: add p-state driver compatibility

Previously, in order to use the power library, it was necessary
for the user to disable the intel_pstate driver by adding
“intel_pstate=disable” to the kernel command line for the system,
which causes the acpi_cpufreq driver to be loaded in its place.

This patch adds the ability for the power library use the intel-pstate
driver.

It adds a new suite of functions behind the current power library API,
and will seamlessly set up the user facing API function pointers to
the relevant functions depending on whether the system is running with
acpi_cpufreq kernel driver, intel_pstate kernel driver or in a guest,
using kvm. The library API and ABI is unchanged.

Signed-off-by: Liang Ma <liang.j.ma@intel.com>
Reviewed-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: David Hunt <david.hunt@intel.com>