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/Kconfig b/drivers/usb/emul/Kconfig
new file mode 100644
index 0000000..ae1ab23
--- /dev/null
+++ b/drivers/usb/emul/Kconfig
@@ -0,0 +1,8 @@
+config USB_EMUL
+	bool "Support for USB device emulation"
+	depends on DM_USB && SANDBOX
+	help
+	  Since sandbox does not have access to a real USB bus, it is possible
+	  to use device emulators instead. This allows testing of the USB
+	  stack on sandbox without needing a real device, or any host machine
+	  USB resources.