ARM: Exynos: switch to using generic cpufreq driver for Exynos4x12

ARM: Exynos: switch to using generic cpufreq driver for Exynos4x12

The new CPU clock type allows the use of generic CPUfreq driver.
Switch Exynos4x12 to using generic cpufreq driver.

This patch also takes care of making ARM_EXYNOS_CPU_FREQ_BOOST_SW
config option depend on cpufreq-dt driver instead of exynos-cpufreq
one and fixes the minor issue present with the old code (support
for 'boost' mode in the exynos-cpufreq driver was enabled for all
supported SoCs even though 'boost' frequency was provided only for
Exynos4x12 ones).

Cc: Tomasz Figa <tomasz.figa@gmail.com>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Cc: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>