cpufreq: mediatek: allow building as a module

The MT8173 cpufreq driver can currently only be built-in, but
it has a Kconfig dependency on the thermal core. THERMAL
can be a loadable module, which in turn makes this driver
impossible to build.

It is nicer to make the cpufreq driver a module as well, so
this patch turns the option in to a 'tristate' and adapts
the dependency accordingly.

The driver has no module_exit() function, so it will continue
to not support unloading, but it can be built as a module
and loaded at runtime now.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 5269e706 (cpufreq: Add ARM_MT8173_CPUFREQ dependency on THERMAL)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>