spl: fit: Warn if FIT contains "fpga" property in config node

Commit 4afc4f37 ("doc: FIT image: Clarify format and simplify
syntax") requires that FPGA images be referenced through the
"loadables" in the config node. This means that "fpga" properties in
config nodes are deprecated.

Given that there are likely FIT images which use "fpga", let's not
break those right away. Print a warning message that such use is
deprecated, and give users a couple of releases to update their

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