arm64: zynqmp: Enable CDC ethernet gadget for zcu100/Ultra96

Ethernet is not present on this board that's why there are two other
options how to wire the board to ethernet. The first is asix_eth usb
host converter which is already enabled by default. The second option is
to use USB CDC/RNDIS ethernet gadget.

This patch is enabling CDC which is working with Linux.
With new bind/unbind command there is no need to call usb_ether_init()
from platform code and use for example these commands:

bind /amba/usb0@ff9d0000/dwc3@fe200000 usb_ether
dhcp
unbind /amba/usb0@ff9d0000/dwc3@fe200000

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