binman: Adjust size of test SPL binary

This is only 3 bytes long which is not enough to hold two symbol values,
needed to test the binman symbols feature. Increase it to 15 bytes.

Using very small regions is useful since we can easily compare them in
tests and errors are fairly easy to diagnose.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/29_x86-rom.dts b/tools/binman/test/29_x86-rom.dts
index 075ede3..d49078e 100644
--- a/tools/binman/test/29_x86-rom.dts
+++ b/tools/binman/test/29_x86-rom.dts
@@ -7,13 +7,13 @@
 	binman {
 		sort-by-pos;
 		end-at-4gb;
-		size = <16>;
+		size = <32>;
 		u-boot {
-			pos = <0xfffffff0>;
+			pos = <0xffffffe0>;
 		};
 
 		u-boot-spl {
-			pos = <0xfffffff7>;
+			pos = <0xffffffeb>;
 		};
 	};
 };