MIPS: sead3: use unflatten_and_copy_device_tree()

we want the device tree to be unflattened into non init memory so it can be
accessed later by, for example, a probing function of a driver module.

Signed-off-by: Qais Yousef <qais.yousef@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6210/