cpufreq: dbx500: Manage cooling device from cpufreq driver

The best place to register the CPU cooling device is from the cpufreq
driver as we would know if all the resources are already available or
not. That's what is done for the cpufreq-dt.c driver as well.

The cpu-cooling driver for dbx500 platform was just (un)registering
with the thermal framework and that can be handled easily by the cpufreq
driver as well and in proper sequence as well.

Get rid of the cooling driver and its its users and manage everything
from the cpufreq driver instead.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>