xtensa: use unflatten_and_copy_device_tree

Use the common unflatten_and_copy_device_tree to copy the built-in FDT
out of init section.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Chris Zankel <chris@zankel.net>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Cc: linux-xtensa@linux-xtensa.org
Acked-by: Grant Likely <grant.likely@linaro.org>