cpufreq: kryo: allow building as a loadable module

Building the kryo cpufreq driver while QCOM_SMEM is a loadable module
results in a link error:

drivers/cpufreq/qcom-cpufreq-kryo.o: In function `qcom_cpufreq_kryo_probe':
qcom-cpufreq-kryo.c:(.text+0xbc): undefined reference to `qcom_smem_get'

The problem is that Kconfig ignores interprets the dependency as met
when the dependent symbol is a 'bool' one. By making it 'tristate',
it will be forced to be a module here, which builds successfully.

Fixes: 46e2856b

 (cpufreq: Add Kryo CPU scaling driver)
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>