MAINTAINERS: Merge the two e500 sections

MAINTAINERS: Merge the two e500 sections

There is currently a "e500" machine section and a "ppce500" device
section in the maintainers file - with some oddities: The wildcard
in the device section also covers the files from the machine section.
And hw/pci-host/ppce500.c is in the device section, while its header
is in the machine section.
This is really quite confusing, and I don't see a reason why we really
need two sections here, so let's simply merge them.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>