MAINTAINERS: Demote sPAPR from "Supported" to "Maintained"

MAINTAINERS: Demote sPAPR from "Supported" to "Maintained"

qemu/KVM on Power is no longer my primary job responsibility, nor Greg
Kurz'.  I still have some time for upstream maintenance, but it's no longer
accurate to say that I'm paid to do so.  Therefore, reduce sPAPR (the
"pseries" machine type) from Supported to Maintained.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>