sandbox: Add serial test

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index 67c1fe6..3f5a634 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -46,4 +46,5 @@
 obj-$(CONFIG_WDT) += wdt.o
 obj-$(CONFIG_AXI) += axi.o
 obj-$(CONFIG_MISC) += misc.o
+obj-$(CONFIG_DM_SERIAL) += serial.o
 endif