powerpc/configs: Re-enable ISO9660_FS as a built-in in 64 bit configs

It turns out cloud-config uses ISO9660 filesystems to inject
configuration data into cloud images. The cloud-config failures when
ISO9660_FS is not enabled are cryptic, and building it in makes
mainline testing easier, so re-enable it.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>