arm64: defconfig: Enable USB_SERIAL_CP210X

Some hardware platforms required CP20x USB to Serial converter
in order to work onboard functionalities like Bluetooth.

An example of such a platform is from Engicam's PX30 (ARM64).

Mark it as module in defconfig.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Link: https://lore.kernel.org/r/20201109181017.206834-10-jagan@amarulasolutions.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>