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>

include/configs/firefly-rk3288.h

@@ -10,7 +10,8 @@
 #define ROCKCHIP_DEVICE_SETTINGS \
 		"stdin=serial,usbkbd\0" \
 		"stdout=serial,vidconsole\0" \
-		"stderr=serial,vidconsole\0"
+		"stderr=serial,vidconsole\0" \
+		"preboot=usb start\0"
 
 #include <configs/rk3288_common.h>