cpufreq: exynos: Show a list of available frequencies

This patch adds freq_attr to show a list of exynos5440 scaling
available frequencies through sysfs. Common exynos driver already
supports this attribute.

Signed-off-by: Jungseok Lee <jays.lee@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>