cpufreq: exynos: Get booting freq value in exynos_cpufreq_init

Boot_freq is for saving booting freq. But exynos_cpufreq_cpu_init
is called in hotplug. If boot_freq is existed in exynos_cpufreq_cpu_init,
boot_freq could be changed.

Signed-off-by: Jonghwan Choi <jhbird.choi@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>