MAINTAINERS: add myself as maintainer for Mac Old World and New World machines

I've unofficially been doing most of the work on the Mac machines for a while
now, so update MAINTAINERS to reflect this. David is still happy to be listed
as a reviewer as per our discussion at KVM forum.

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

MAINTAINERS

@@ -976,7 +976,8 @@ F: hw/ppc/mpc8544ds.c
 F: hw/ppc/mpc8544_guts.c
 
 New World (mac99)
-M: David Gibson <david@gibson.dropbear.id.au>
+M: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
+R: David Gibson <david@gibson.dropbear.id.au>
 L: qemu-ppc@nongnu.org
 S: Odd Fixes
 F: hw/ppc/mac_newworld.c
@@ -994,7 +995,8 @@ F: include/hw/input/adb*
 F: pc-bios/qemu_vga.ndrv
 
 Old World (g3beige)
-M: David Gibson <david@gibson.dropbear.id.au>
+M: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
+R: David Gibson <david@gibson.dropbear.id.au>
 L: qemu-ppc@nongnu.org
 S: Odd Fixes
 F: hw/ppc/mac_oldworld.c