sandbox: Enable fdtdec tests

Enable fdtdec tests on sandbox configurations so that they can be run to
validate the fdtdec implementation.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox64_defconfig b/configs/sandbox64_defconfig
index da4bdce..c04ecd9 100644
--- a/configs/sandbox64_defconfig
+++ b/configs/sandbox64_defconfig
@@ -194,6 +194,7 @@
 CONFIG_TPM=y
 CONFIG_LZ4=y
 CONFIG_ERRNO_STR=y
+CONFIG_TEST_FDTDEC=y
 CONFIG_UNIT_TEST=y
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y