MAINTAINERS: PPC: Remove myself

I haven't really been maintaining any PowerPC code for quite a while now,
so let's reflect reality: David does all the work and embedded PPC is in
"Odd Fixes" state rather than supported now.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>