cpufreq: exynos: Convert exynos-cpufreq to platform driver

To make the driver multiplatform-friendly, unconditional initialization
in an initcall is replaced with a platform driver probed only if
respective platform device is registered.

Tested at: Exynos4210 (TRATS) and Exynos4412 (TRATS2)

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Sachin Kamat <sachin.kamat@linaro.org>
Tested-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>