cpufreq: exynos: Disable on multiplatform build

The current exynos cpufreq drivers are not multiplatform compliant
and give build errors as they refer to header files from machine
directory. Work to migrate them to generic cpufreq framework is
under way. Till such time disable the build on multiplatform so
that other multiplatform ready features get tested.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>