gemini: Activate serial USB console on the DNS-313

gemini: Activate serial USB console on the DNS-313

This brings up a serial console on the USB device port of
the DNS-313 by:

- Activating the usbgadget feature
- Selecting the usbgadget-acm package
- Adding an inittab that opens a console at ttyGS0 which is
  the device side of ttyACMn of a connected host

Link: http://patchwork.ozlabs.org/project/openwrt/patch/20250403-dns313-usb-serial-v2-4-d84de8e86931@linaro.org/
Cc: Chuanhong Guo <gch981213@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>