cpufreq-dt: make scaling_boost_freqs sysfs attr available when boost is enabled

Make scaling_boost_freqs sysfs attribute is available when
cpufreq-dt driver is used and boost support is enabled.

Suggested-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>