cpufreq: Support per-policy performance boost

The boost control currently applies to the whole system.  However, users
may prefer to boost a subset of cores in order to provide prioritized
performance to workloads running on the boosted cores.

Enable per-policy boost by adding a 'boost' sysfs interface under each
policy path.  This can be found at:

	/sys/devices/system/cpu/cpufreq/policy<*>/boost

Same to the global boost switch, writing 1/0 to the per-policy 'boost'
enables/disables boost on a cpufreq policy respectively.

The user view of global and per-policy boost controls should be:

1. Enabling global boost initially enables boost on all policies, and
per-policy boost can then be enabled or disabled individually, given that
the platform does support so.

2. Disabling global boost makes the per-policy boost interface illegal.

Signed-off-by: Jie Zhan <zhanjie9@hisilicon.com>
Reviewed-by: Wei Xu <xuwei5@hisilicon.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/linux/cpufreq.h

@@ -141,6 +141,9 @@ struct cpufreq_policy {
 	 */
 	bool			dvfs_possible_from_any_cpu;
 
+	/* Per policy boost enabled flag. */
+	bool			boost_enabled;
+
 	 /* Cached frequency lookup from cpufreq_driver_resolve_freq. */
 	unsigned int cached_target_freq;
 	unsigned int cached_resolved_idx;