mips: octeon: convert to use unflatten_and_copy_device_tree

The octeon FDT code can be simplified by using
unflatten_and_copy_device_tree function. This removes all accesses to
FDT header data by the arch code.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Tested-by: Grant Likely <grant.likely@linaro.org>