sandbox: Add a test for IRQ

Add a simple sandbox test for this uclass.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index 129ccb3..a268783 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -25,6 +25,7 @@
 obj-$(CONFIG_DM_HWSPINLOCK) += hwspinlock.o
 obj-$(CONFIG_DM_I2C) += i2c.o
 obj-$(CONFIG_SOUND) += i2s.o
+obj-y += irq.o
 obj-$(CONFIG_LED) += led.o
 obj-$(CONFIG_DM_MAILBOX) += mailbox.o
 obj-$(CONFIG_DM_MMC) += mmc.o