spl: fit: Support loading FPGA images from list of "loadables"

Commit 4afc4f37 ("doc: FIT image: Clarify format and simplify
syntax") and delegated FPGA images to be added via the list of
"loadables" in lieu of the "fpga" property. Now actually implement
this in code.

Note that the "compatible" property is ignored for the time being, as
implementing "compatible" loading is beyond the scope of this change.
However, "u-boot,fpga-legacy" is accepted without warning.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>