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/drivers/usb/Kconfig b/drivers/usb/Kconfig
index a4414ef..637ef3d 100644
--- a/drivers/usb/Kconfig
+++ b/drivers/usb/Kconfig
@@ -51,6 +51,8 @@
 
 source "drivers/usb/host/Kconfig"
 
+source "drivers/usb/emul/Kconfig"
+
 config USB_STORAGE
 	bool "USB Mass Storage support"
 	---help---