binman: fit: Write the compatible string to configuration

FIT allows the FDT's root-node compatible string to be placed in a
configuration node to simplify and speed up finding the best match for
booting.

Add a new property to support this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entries.rst b/tools/binman/entries.rst
index dcacd29..fd8edc6 100644
--- a/tools/binman/entries.rst
+++ b/tools/binman/entries.rst
@@ -939,6 +939,7 @@
             firmware = "atf";
             loadables = "uboot";
             fdt = "fdt-SEQ";
+            fit,compatible;    // optional
         };
     };
 
@@ -948,6 +949,10 @@
 Note that if no devicetree files are provided (with '-a of-list' as above)
 then no nodes will be generated.
 
+The 'fit,compatible' property (if present) is replaced with the compatible
+string from the root node of the devicetree, so that things work correctly
+with FIT's configuration-matching algortihm.
+
 Generating nodes from an ELF file (split-elf)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~