cpufreq: exynos4x12: Change L0 driver data to CPUFREQ_BOOST_FREQ

Add a special driver data flag (CPUFREQ_BOOST_FREQ) to indicate
a frequency that can be only enabled for BOOST mode.

This frequency will be used only for limited time, since running with
it for too long may cause the target device to overheat.

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