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>