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.

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>

Changes in v7:
-none

Changes in v6:
-none

Changes in v5:
-none

Changes in v4:
-move USB_FUNCTION_ROCKUSB to drivers/usb/gadget/Kconfig
-modify the dependence

Changes in v3:
-fix comment from Simon and Lukasz
-fix checkpactch error

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