MAINTAINERS: Add section for cpuidle-psci PM domain

The cpuidle-psci-domain.c is not listed in the section for the cpuidle
driver for ARM PSCI.

From discussions at LKML, Lorenzo and Sudeep prefer to add a separate
section for it, so do that and add myself as the maintainer for that
part.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>