arm64: zynqmp: Start usb ethernet gadget automatically

If only usb ethernet gadget is enabled it can start automatically.
If more gagdets are enabled usb ethernet gadget can be bind by
"bind /amba/usb1@ff9e0000/dwc3@fe300000 usb_ether" (on zcu100)

Signed-off-by: Michal Simek <michal.simek@xilinx.com>