usb: rockchip: add rockusb command

this patch add rockusb command. the usage is
rockusb <USB_controller> <devtype> <dev[:part]>
e.g. rockusb 0 mmc 0

Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index c033223..83dc778 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -914,6 +914,14 @@
 	help
 	  Enables the command "sdp" which is used to have U-Boot emulating the
 	  Serial Download Protocol (SDP) via USB.
+config CMD_ROCKUSB
+	bool "rockusb"
+	depends on USB_FUNCTION_ROCKUSB
+	help
+          Rockusb protocol is widely used by Rockchip SoC based devices. It can
+	  read/write info, image to/from devices. This enable rockusb command
+	  support to communication with rockusb device. for more detail about
+	  this command, please read doc/README.rockusb.
 
 config CMD_USB_MASS_STORAGE
 	bool "UMS usb mass storage"