MAINTAINERS: Add an entry for the PM core

Add a MAINTAINERS entry for the PM core with myself as the maintainer
and linux-pm as the mailing list.

This actually documents the current state of things.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>