dm: rtc: Add tests for real-time clocks

Add some simple tests to verify that the RTC uclass works correctly in
U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index fd9e29f..a0cc2c6 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -21,6 +21,7 @@
 obj-$(CONFIG_DM_GPIO) += gpio.o
 obj-$(CONFIG_DM_I2C) += i2c.o
 obj-$(CONFIG_DM_PCI) += pci.o
+obj-$(CONFIG_DM_RTC) += rtc.o
 obj-$(CONFIG_DM_SPI_FLASH) += sf.o
 obj-$(CONFIG_DM_SPI) += spi.o
 obj-$(CONFIG_DM_USB) += usb.o