binman: Allow different operations in FIT generator nodes

At present we only support expanding out FDT nodes. Make the operation
into an @operation property, so that others can be supported.

Re-arrange and tidy up the documentation so that it has separate
headings for each topic.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/224_fit_bad_oper.dts b/tools/binman/test/224_fit_bad_oper.dts
new file mode 100644
index 0000000..cee801e
--- /dev/null
+++ b/tools/binman/test/224_fit_bad_oper.dts
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		fit {
+			description = "test-desc";
+			#address-cells = <1>;
+			fit,fdt-list = "of-list";
+
+			images {
+				@fdt-SEQ {
+					fit,operation = "unknown";
+					description = "fdt-NAME.dtb";
+					type = "flat_dt";
+					compression = "none";
+				};
+			};
+		};
+		fdtmap {
+		};
+	};
+};