cpufreq: exynos: Extend Exynos cpufreq driver to support boost

The cpufreq_driver's boost_supported flag is true only when boost
support is explicitly enabled. Boost related attributes are exported
only under the same condition.

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>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>