powercap/dtpm_cpu: Add exit function

Now that we can destroy the hierarchy, the code must remove what it
had put in place at the creation. In our case, the cpu hotplug
callbacks.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20220130210210.549877-6-daniel.lezcano@linaro.org