ARM: dts: rockchip: enable usbotg for Popemtal-rk3288 board

Enable USB OTG port on RK3288 Popmetal board, So we can run
some usb gadget functions like Android adb on this board.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>