spl: fit: implement recording of loadables into /fit-images

If a FDT was loaded (e.g. to append it to U-Boot image), we store it's
address and record information for all loadables into this FDT.  This
allows us to easily keep track of images for multiple privilege levels
(e.g. with ATF) or of firmware images preloaded into temporary
locations (e.g. PMU firmware that may overlap the SPL stage).

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