dm: devres: Add tests

The devres functionality has very few users in U-Boot, but it still should
have tests. Add a few basic tests of the main functions.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index a268783..85cc0f7 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -18,6 +18,7 @@
 obj-$(CONFIG_BOARD) += board.o
 obj-$(CONFIG_DM_BOOTCOUNT) += bootcount.o
 obj-$(CONFIG_CLK) += clk.o clk_ccf.o
+obj-$(CONFIG_DEVRES) += devres.o
 obj-$(CONFIG_VIDEO_MIPI_DSI) += dsi_host.o
 obj-$(CONFIG_DM_ETH) += eth.o
 obj-$(CONFIG_FIRMWARE) += firmware.o