cpufreq: scpi: Use auto-registration of thermal cooling device

Use the CPUFREQ_IS_COOLING_DEV flag to allow cpufreq core to
automatically register as a thermal cooling device.

This allows removal of boiler plate code from the driver.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>