common/board_f: Respect original FDT size while relocating

While relocating FDT we reserve some memory for the new FDT and
set the size of the FDT with that respect. But FDT may be placed
at the end of the RAM leading to memory access beyond it.
Fix this by copying exact FDT size bytes, not the reserved size.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Simon Glass <sjg@chromium.org>