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

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>