PM / Domains: Add DT file to MAINTAINERS

Add the power_domain.txt DT file to MAINTAINERS, otherwise
get_maintainers.pl doesn't pick the right set of maintainers for changes
to the DT file.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>