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

Add a special help message pointing to the relevant README.
Signed-off-by: NSimon Glass <sjg@chromium.org>

arch/arm/dts/sunxi-u-boot.dtsi

@@ -48,6 +48,7 @@
 					entry = <0x44000>;
 #endif
 					atf-bl31 {
+						missing-msg = "atf-bl31-sunxi";
 					};
 				};
 

tools/binman/missing-blob-help

@@ -9,3 +9,7 @@
 atf-bl31:
 See the documentation for your board. You may need to build ARM Trusted
 Firmware and build with BL31=/path/to/bl31.bin
+
+atf-bl31-sunxi:
+Please read the section on ARM Trusted Firmware (ATF) in
+board/sunxi/README.sunxi64