rockchip: firefly: Add "usb start" to auto-start USB device

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@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>