MAINTAINERS: Add David Gibson as ppc maintainer

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: NDavid Gibson <david@gibson.dropbear.id.au>
Reviewed-by: NAlexander Graf <agraf@suse.de>
Acked-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>

MAINTAINERS

@@ -165,6 +165,7 @@ F: hw/openrisc/
 F: tests/tcg/openrisc/
 
 PowerPC
+M: David Gibson <david@gibson.dropbear.id.au>
 M: Alexander Graf <agraf@suse.de>
 L: qemu-ppc@nongnu.org
 S: Maintained