MAINTAINERS: Remove myself as reviewer

MAINTAINERS: Remove myself as reviewer

I've now well and truly moved on from ppc and qemu maintenance.  I'm
occupied with other things and am pretty much just ignoring mails on these
topics I'm CCed on.  Time to remove myself.

I'm still listed as a reviewer for Device Tree, I'll keep this for now,
since I do have some interest and it's lower volume.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>