dm: usb: Add tests for the USB uclass

This adds a simple test for probing and a functional test using the flash
stick emulator, which tests a large chunk of the USB stack.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index a2e2d23..fd9e29f 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -23,4 +23,5 @@
 obj-$(CONFIG_DM_PCI) += pci.o
 obj-$(CONFIG_DM_SPI_FLASH) += sf.o
 obj-$(CONFIG_DM_SPI) += spi.o
+obj-$(CONFIG_DM_USB) += usb.o
 endif