spl: fit: implement fdt_record_loadable

During the loading of more complex FIT images (e.g. when the invoked
next stage needs to find additional firmware for a power-management
core... or if there are multiple images for different privilege levels
started in parallel), it is helpful to create a record of what images
are loaded where: if a FDT is loaded for one of the next stages, it
can be used to convey the status and location of loadables.

This adds a fdt_record_loadable() function that can be invoked to
record the status of each loadable below the /fit-images path.

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