arm64: defconfig: Enable USB gadget with configfs

USB OTG connections are pretty common for embedded and development
boards, for example to have networking or serial access to the device.
Build as a module the USB gadget configfs options so the defconfig can
be used in such development configurations.

Link: https://lore.kernel.org/r/20200817065654.4419-1-krzk@kernel.org
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>