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.

Previously (when exynos-cpufreq driver was used with boost
functionality) ARM_EXYNOS_CPU_FREQ_BOOST_SW config option
(which enabled boost functionality) selected EXYNOS_THERMAL
one. After switching Exynos4x12 platforms to use cpufreq-dt
driver boost support is enabled in the cpufreq-dt driver
itself (because there are turbo OPPs defined in the board's
DTS file). However we still would like to allow enabling
boost support only if thermal support is also enabled for
Exynos platforms. To achieve this make ARCH_EXYNOS config
option select THERMAL and EXYNOS_THERMAL ones.

Please also note that the switch to use the generic cpufreq-dt
driver 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: Thomas Abraham <thomas.ab@samsung.com>
Cc: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>