openrisc: use unflatten_and_copy_device_tree

Use the common unflatten_and_copy_device_tree to copy the built-in FDT
out of init section. This moves the copy later in the boot, but there
do not appear to be any references to strings in the FDT before the copy.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: linux@lists.openrisc.net