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>

doc/uImage.FIT/multi-with-fpga.its

@@ -29,6 +29,7 @@
 			arch = "arm";
 			compression = "none";
 			load = <0x30000000>;
+			compatible = "u-boot,fpga-legacy"
 			hash-1 {
 				algo = "md5";
 			};
@@ -61,7 +62,7 @@
 			description = "Linux with fpga";
 			kernel = "linux_kernel";
 			fdt = "fdt-1";
-			fpga = "fpga";
+			loadables = "fpga";
 		};
 	};
 };