powercap/drivers/dtpm: Fix root node initialization

The root node is not set to NULL when the dtpm root node is
removed. Consequently, it is not possible to create a new root
as it is already set.

Set the root node to NULL when the last node is removed.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>