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/MAINTAINERS b/MAINTAINERS
index baf2beb..b231610 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -492,6 +492,7 @@
 M:	Eddie Cai <eddie.cai.linux@gmail.com>
 S:	Maintained
 F:	drivers/usb/gadget/f_rockusb.c
+F:	cmd/rockusb.c
 
 VIDEO
 M:	Anatolij Gustschin <agust@denx.de>