test: rng: Add basic test for random number generator(rng) uclass

Add a unit test for testing the rng uclass functionality using the
sandbox rng driver.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-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 a268783..201e2b0 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -68,4 +68,5 @@
 obj-$(CONFIG_DMA) += dma.o
 obj-$(CONFIG_DM_MDIO) += mdio.o
 obj-$(CONFIG_DM_MDIO_MUX) += mdio_mux.o
+obj-$(CONFIG_DM_RNG) += rng.o
 endif