spl: add a fdt_addr field to spl_image_info

When loading a full U-Boot with detached device-tree using the SPL FIT
backend, we should store the address of the FDT loaded as part of the
SPL image info: this allows us to fixup the FDT with additional info
we may want to propagate onward.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>