dtpm/soc/rk3399: Add the ability to unload the module

The dtpm hierarchy can now be removed with the
dtpm_destroy_hierarchy() function. Add the module_exit() callback so
the module can be unloaded by removing the previously created
hierarchy.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20220130210210.549877-7-daniel.lezcano@linaro.org