dm: spl: Generate u-boot-spl-dtb.bin only when enabled

At present this file is generated even when device tree is not enabled in
SPL. Avoid this, since this file serves no purpose in that case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>