cpufreq: exynos: Fix the compile error

Commit 7da83a80 ("ARM: EXYNOS: Migrate Exynos specific macros from
plat to mach") which lands in samsung tree causes build breakage
for cpufreq-exynos like following:

drivers/cpufreq/exynos-cpufreq.c: In function 'exynos_cpufreq_probe':
drivers/cpufreq/exynos-cpufreq.c:166:2: error: implicit declaration of function 'soc_is_exynos4210'
[-Werror=implicit-function-declaration]
drivers/cpufreq/exynos-cpufreq.c:168:2: error: implicit declaration of function 'soc_is_exynos4212'
[-Werror=implicit-function-declaration]
drivers/cpufreq/exynos-cpufreq.c:168:2: error: implicit declaration of function 'soc_is_exynos4412'
[-Werror=implicit-function-declaration]
drivers/cpufreq/exynos-cpufreq.c:170:2: error: implicit declaration of function 'soc_is_exynos5250'
[-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors
make[2]: *** [drivers/cpufreq/exynos-cpufreq.o] Error 1
make[2]: *** Waiting for unfinished jobs....
drivers/cpufreq/exynos4x12-cpufreq.c: In function 'exynos4x12_set_clkdiv':
drivers/cpufreq/exynos4x12-cpufreq.c:118:2: error: implicit declaration of function 'soc_is_exynos4212'
[-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors
make[2]: *** [drivers/cpufreq/exynos4x12-cpufreq.o] Error 1
make[1]: *** [drivers/cpufreq] Error 2

This fixes above error with getting SoC information via
of_machine_is_compatible() instead of soc_is_exynosXXXX().

Suggested-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Jonghwan Choi <jhbird.choi@samsung.com>
[kgene.kim@samsung.com: fixed typo and modified as per Viresh's suggestion]
[kgene.kim@samsung.com: Rafael agreed]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>