MAINTAINERS: Orphan obscure ppc platforms

MAINTAINERS: Orphan obscure ppc platforms

There are a nunber of old embedded ppc machine types which have been little
changed and in "Odd Fixes" state for a long time.  With both myself and
Greg Kurz moving toward other areas, we no longer have the capacity to
keep reviewing and maintaining even the rare patches that come in for those
platforms.

Therefore, remove our names as reviewers and mark these platforms as
orphaned.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>