cpufreq: Add ARM_MT8173_CPUFREQ dependency on THERMAL

If ARM_MT8173_CPUFREQ is configured, and THERMAL is configured as module,
the following build error is seen for arm:allmodconfig and
arm64:allmodconfig.

drivers/built-in.o: In function `mtk_cpufreq_ready':
:(.text+0x32a20c): undefined reference to `of_cpufreq_cooling_register'
drivers/built-in.o: In function `mtk_cpufreq_exit':
:(.text+0x32a420): undefined reference to `cpufreq_cooling_unregister'

The fix is similar to CPUFREQ_DT, but more restrictive since
ARM_MT8173_CPUFREQ can not be built as module.

Fixes: 1453863f ("cpufreq: mediatek: Add MT8173 cpufreq driver")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>