cpufreq: exynos: Show list of available frequencies

Add freq_attr attribute to show list of available frequencies.

Signed-off-by: Donggeun Kim <dg77.kim@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: KyungMin Park <kyungmin.park@samsung.com>
Signed-off-by: Inderpal Singh <inderpal.singh@linaro.org>
Reviewed-by: Amit Daniel Kachhap<amit.daniel@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>