binman: Support adding files

In some cases it is useful to add a group of files to the image and be
able to access them at run-time. Of course it is possible to generate
the binman config file with a set of blobs each with a filename. But for
convenience, add an entry type which can do this.

Add required support (for adding nodes and string properties) into the
state module.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/87_files_no_pattern.dts b/tools/binman/test/87_files_no_pattern.dts
new file mode 100644
index 0000000..0cb5b46
--- /dev/null
+++ b/tools/binman/test/87_files_no_pattern.dts
@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: GPL-2.0+
+/dts-v1/;
+
+/ {
+	binman {
+		files {
+			compress = "none";
+			require-matches;
+		};
+	};
+};