dm: test: Disable the fdt_offset test with livetree

We cannot run this test with livetree since it uses device tree offsets.
Mark it as flat tree only.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/test-fdt.c b/test/dm/test-fdt.c
index 3048a7b..987a265 100644
--- a/test/dm/test-fdt.c
+++ b/test/dm/test-fdt.c
@@ -265,4 +265,5 @@
 
 	return 0;
 }
-DM_TEST(dm_test_fdt_offset, DM_TESTF_SCAN_PDATA | DM_TESTF_SCAN_FDT);
+DM_TEST(dm_test_fdt_offset,
+	DM_TESTF_SCAN_PDATA | DM_TESTF_SCAN_FDT | DM_TESTF_FLAT_TREE);