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>