mkimage: Report information about fpga

Add FIT_FPGA_PROP that user can identify an optional
entry for fpga.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/uImage.FIT/source_file_format.txt b/doc/uImage.FIT/source_file_format.txt
index 9c527c3..3f54180 100644
--- a/doc/uImage.FIT/source_file_format.txt
+++ b/doc/uImage.FIT/source_file_format.txt
@@ -236,6 +236,7 @@
   |- kernel = "kernel sub-node unit name"
   |- ramdisk = "ramdisk sub-node unit name"
   |- fdt = "fdt sub-node unit-name"
+  |- fpga = "fpga sub-node unit-name"
   |- loadables = "loadables sub-node unit-name"
 
 
@@ -251,6 +252,8 @@
     "fdt type").
   - setup : Unit name of the corresponding setup binary (used for booting
     an x86 kernel). This contains the setup.bin file built by the kernel.
+  - fpga : Unit name of the corresponding fpga bitstream blob
+    (component image node of a "fpga type").
   - loadables : Unit name containing a list of additional binaries to be
     loaded at their given locations.  "loadables" is a comma-separated list
     of strings. U-Boot will load each binary at its given start-address.