cpufreq: fix EXYNOS drivers selection

* remove superfluous pr_debug() call from exynos_cpufreq_init()
  (init errors are always logged anyway)
* add dummy per-SoC type init functions to exynos-cpufreq.h
* make per-SoC type cpufreq config options selectable
* make CONFIG_ARM_EXYNOS_CPUFREQ config option invisible to user and
  automatically enable it when needed

This patch fixes following issues:
* EXYNOS per-SoC type cpufreq support (i.e. exynos4210-cpufreq.c) being
  always built if given SoC support was enabled (i.e. CPU_EXYNOS4210),
  even if common EXYNOS cpufreq support was disabled
* inability to select cpufreq for each SoC type separately (it could
  be only enabled/disabled for all SoCs for which support was enabled)
* EXYNOS5440 cpufreq support was always enabled when EXYNOS5440
  support was enabled and couldn't be disabled

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>