xtensa: Flat DeviceTree copy not future-safe

flat DT copy code calls bootmem allocator with @align = 0.
This is probably OK with legacy allocator which xtensa uses right now,
but this will panic right away with memblock allocator

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Marc Gauthier <marc@tensilica.com>
Cc: linux-xtensa@linux-xtensa.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>