cpufreq: dbx500: add a Kconfig symbol

Moving the cooling code into the cpufreq driver caused a possible build failure
when the cpu_thermal helper code is a loadable module or disabled:

drivers/cpufreq/dbx500-cpufreq.o: In function `dbx500_cpufreq_ready':
dbx500-cpufreq.c:(.text.dbx500_cpufreq_ready+0x4): undefined reference to `cpufreq_cooling_register'

This adds the same dependency that we have in other cpufreq drivers,
forcing the driver to be disabled when we can't possibly link it.

Fixes: 19678ffb (cpufreq: dbx500: Manage cooling device from cpufreq driver)
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>