powercap/drivers/dtpm: Add the experimental label to the option description

The DTPM framework will evolve in the next cycles. Let's add a
temporary EXPERIMENTAL tag to the option so users will be aware
the API may change over time.

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