dm: usb: sandbox: Add an emulator for USB flash devices

This emulator supports USB enumeration and allows a local file to be provided
as the contents of the emulated flash stick. U-Boot can then use the file as
it would a normal device, with all access passing through the usb_stor layer
and the USB stack.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/include/dm/uclass-id.h b/include/dm/uclass-id.h
index 3d62dbb..fddfd35 100644
--- a/include/dm/uclass-id.h
+++ b/include/dm/uclass-id.h
@@ -44,6 +44,7 @@
 	UCLASS_USB,		/* USB bus */
 	UCLASS_USB_HUB,		/* USB hub */
 	UCLASS_USB_DEV_GENERIC,	/* USB generic device */
+	UCLASS_MASS_STORAGE,	/* Mass storage device */
 
 	UCLASS_COUNT,
 	UCLASS_INVALID = -1,