dm: usb: sandbox: Add a uclass for USB device emulation

With sandbox we want to be able to emulate USB devices so that we can test
the USB stack. Add a uclass to support this. It implements the same
operations as a normal USB device driver, but in this case passes them on
to an emulation driver.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/Makefile b/Makefile
index 0f7d583..dc25f70 100644
--- a/Makefile
+++ b/Makefile
@@ -651,6 +651,7 @@
 libs-$(CONFIG_SYS_FSL_DDR) += drivers/ddr/fsl/
 libs-y += drivers/serial/
 libs-y += drivers/usb/dwc3/
+libs-y += drivers/usb/emul/
 libs-y += drivers/usb/eth/
 libs-y += drivers/usb/gadget/
 libs-y += drivers/usb/gadget/udc/