doc: FIT image: Update FPGA example to make use of "loadables"

The new correct way to load an FPGA image is to declare it in the list
of "loadables". multi-with-fpga.its used the now deprecated "fpga"
property. Since this example most likely intended to use u-boot's
generic FPGA loading code, compatible = "u-boot,fpga-legacy" is also
appropriate here.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>