dt-bindings: 83xx-512x-pci: Drop cell-index property

28eac2b7 ("powerpc/fsl: Remove cell-index from PCI nodes"),
and for now it is still not used, drop it from doc.

Cc: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Rob Herring <robh@kernel.org>