I've been de facto co-maintainer of all ppc target related code for some
time.  Alex Graf isworking on other things and doesn't have a whole lot of
time for qemu ppc maintainership.  So, update the MAINTAINERS file to
reflect this.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexander Graf <agraf@suse.de>
Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>