cpufreq: Add support for per-policy driver data

Drivers supporting multiple clusters or multiple 'struct cpufreq_policy'
instances may need to keep per-policy data. If the core doesn't provide support
for that, they might do it in the most unoptimized way: 'per-cpu' data.

This patch adds another field in struct cpufreq_policy: 'driver_data'. It isn't
accessed by core and is for driver's internal use only.

Tested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>