[U-Boot] [U-Boot, v6, 3/3] rockchip: firefly: Add "usb start" to auto-start USB device

[Philipp Tomsich]

> Add "preboot=usb start" to ROCKCHIP_DEVICE_SETTINGS,you don't
> need to input "usb start" in command line of u-boot console,it
> can auto-start the USB device,after that usb keyboard can work.
> 
> Signed-off-by: Leo Wen <leo.wen at rock-chips.com>
> Acked-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> ---
>  include/configs/firefly-rk3288.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 

Reviewed-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>