doc: FIT image: Introduce "u-boot, fpga-legacy" property

Commit 4afc4f37 ("doc: FIT image: Clarify format and simplify
syntax") introduced a "compatible" property for loadable images.
It did not define its contents. Use "u-boot,fpga-legacy" compatible
string to specify that fpga_load() should be used to load the image.

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