usb: rockchip: add the rockusb gadget

this patch implement rockusb protocol on the device side. this is based on
USB download gadget infrastructure. the rockusb function implements the rd,
wl, rid commands. it can work with rkdeveloptool

Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
index 102a63b..c387f5e 100644
--- a/drivers/usb/gadget/Kconfig
+++ b/drivers/usb/gadget/Kconfig
@@ -134,6 +134,14 @@
 	  allows to download images into memory and execute (jump to) them
 	  using the same protocol as implemented by the i.MX family's boot ROM.
 
+config USB_FUNCTION_ROCKUSB
+        bool "Enable USB rockusb gadget"
+        help
+          Rockusb protocol is widely used by Rockchip SoC based devices. It can
+          read/write info, image to/from devices. This enables the USB part of
+          the rockusb gadget.for more detail about Rockusb protocol, please see
+          doc/README.rockusb
+
 endif # USB_GADGET_DOWNLOAD
 
 config USB_ETHER