binman: sunxi: Add help message for missing sunxi ATF BL31

Add a special help message pointing to the relevant README.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/dts/sunxi-u-boot.dtsi b/arch/arm/dts/sunxi-u-boot.dtsi
index 1d1c369..c97943b 100644
--- a/arch/arm/dts/sunxi-u-boot.dtsi
+++ b/arch/arm/dts/sunxi-u-boot.dtsi
@@ -48,6 +48,7 @@
 					entry = <0x44000>;
 #endif
 					atf-bl31 {
+						missing-msg = "atf-bl31-sunxi";
 					};
 				};