cpufreq: exynos: Use simple approach to asses if cpu cooling can be used

Commit: e725d26c provided possibility to
use device tree to asses if cpu can be used as cooling device. Since the
code was somewhat awkward, simpler approach has been proposed.

Test HW: Exynos 4412 - Odroid U3.

Suggested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>