mips: lantiq: copy built-in DTB out of init section

The existing code is buggy because built-in DTBs are in init memory.
Fix this by using the 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>