acpi-cpufreq: Adjust the code to use the common boost attribute

Modify acpi-cpufreq's hardware-based boost solution to work with the
common cpufreq boost framework.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
[rjw: Subject and changelog]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>