cpufreq: mediatek: fix build error

The recently added mt8173 cpufreq driver relies on the cpu topology
that is always present on ARM64 but optional on ARM32:

drivers/cpufreq/mt8173-cpufreq.c: In function 'mtk_cpufreq_init':
drivers/cpufreq/mt8173-cpufreq.c:441:30: error: 'cpu_topology' undeclared (first use in this function)
  cpumask_copy(policy->cpus, &cpu_topology[policy->cpu].core_sibling);

This refines the Kconfig dependencies so that we can still build on
ARM32, but only if COMPILE_TEST is selected and the CPU topology
code is present.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>