At Alex Graf's request I'm now acting as sub-maintainer for the sPAPR
(-machine pseries) code.  This updates MAINTAINERS accordingly.

While we're at it, change the label to mention pseries since that's the
actual name of the machine type, even if most of the C files use the sPAPR
name.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>