defconfig: arm64: Enable USBNET driver

This change enables USBNET driver which allows USB ethernet class
driver to register USB ethernet device as network device with Linux
Network Stack and allows USB IOs with USB Host Controller driver.

Change-Id: Iba036ce43656979df0b9ea228b57bc980c4c53bc
Signed-off-by: Mayank Rana <mrana@codeaurora.org>