binman: Expand the external FIT test a little

At present this does not check that the external data is in the expected
place. Use a non-zero offset for the external data and check it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/162_fit_external.dts b/tools/binman/test/162_fit_external.dts
index 19518e0..6f2a629 100644
--- a/tools/binman/test/162_fit_external.dts
+++ b/tools/binman/test/162_fit_external.dts
@@ -10,7 +10,7 @@
 		u-boot {
 		};
 		fit {
-			fit,external-offset = <0>;
+			fit,external-offset = <0x400>;
 			description = "test-desc";
 			#address-cells = <1>;