Documentation/powercap/dtpm: Add documentation for dtpm

The dynamic thermal and power management is a technique to dynamically
adjust the power consumption of different devices in order to ensure a
global thermal constraint.

An userspace daemon is usually monitoring the temperature and the
power to take immediate action on the device.

The DTPM framework provides an unified API to userspace to act on the
power.

Document this framework.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>