powercap/dtpm: Destroy hierarchy function

The hierarchy creation function exits but without a destroy hierarchy
function. Due to that, the modules creating the hierarchy can not be
unloaded properly because they don't have an exit callback.

Provide the dtpm_destroy_hierarchy() function to remove the previously
created hierarchy.

The function relies on all the release mechanisms implemented by the
underlying powercap framework.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20220130210210.549877-4-daniel.lezcano@linaro.org