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

The existing code is buggy because built-in DTBs are in init memory.
It is also broken because the reserved bootmem was then freed after
unflattening, but the unflattened tree points to data in the flat tree.
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>