binman: Add support for ATF BL31

Add an entry for ARM Trusted Firmware's 'BL31' payload, which is the
device's main firmware. Typically this is U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/169_atf_bl31.dts b/tools/binman/test/169_atf_bl31.dts
new file mode 100644
index 0000000..2b7547d
--- /dev/null
+++ b/tools/binman/test/169_atf_bl31.dts
@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		size = <16>;
+
+		atf-bl31 {
+			filename = "bl31.bin";
+		};
+	};
+};