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/emul/Makefile b/drivers/usb/emul/Makefile
new file mode 100644
index 0000000..f75bbd8
--- /dev/null
+++ b/drivers/usb/emul/Makefile
@@ -0,0 +1,8 @@
+#
+# (C) Copyright 2015 Google, Inc
+# Written by Simon Glass <sjg@chromium.org>
+#
+# SPDX-License-Identifier:	GPL-2.0+
+#
+
+obj-$(CONFIG_USB_EMUL) += usb-emul-uclass.o