cpufreq: intel_pstate: New sysfs entry to control HWP boost

A new attribute is added to intel_pstate sysfs to enable/disable
HWP dynamic performance boost.

Reported-by: Mel Gorman <mgorman@techsingularity.net>
Tested-by: Giovanni Gherdovich <ggherdovich@suse.cz>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>