sectors field is not being updated when reading fdt from fit image. Because of
this size_of(u-boot.bin) is being read when reading fdt. Fixing it by updating
the sectors field properly.

Tested-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>