MAINTAINERS: Add DTPM subsystem maintainer

The DTPM framework is a new framework allowing to do power limitation
on devices by using different techniques. Those will be added,
improved and complexified.

The framework falls under the power management umbrella, it is more
traffic to handle for Rafael.

Add myself as the maintainer of the DTPM so I can help by taking care
of the changes for this framework.

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