dm: test: Add tests for the clk uclass

Add tests of each API call using a sandbox clock device.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index 19ad2fb..7947545 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -15,6 +15,7 @@
 # subsystem you must add sandbox tests here.
 obj-$(CONFIG_UT_DM) += core.o
 ifneq ($(CONFIG_SANDBOX),)
+obj-$(CONFIG_CLK) += clk.o
 obj-$(CONFIG_DM_ETH) += eth.o
 obj-$(CONFIG_DM_GPIO) += gpio.o
 obj-$(CONFIG_DM_I2C) += i2c.o