MAINTAINERS: Mark powerpc Cell as orphaned

Arnd is no longer actively maintaining Cell, mark it as orphan.

Also drop the dead developerworks link.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240726123322.1165562-1-mpe@ellerman.id.au