boards: beagle: beagleconnect_freedom: Disable 2.4g ieee802154

- A lot of zephyr networking samples (zperf, http_server, etc) will
  complain if both 2.4g and subg ieee802154 are enabled simultaneously.
- This leads to a lot of new people getting confused with the network code
  not working without any clear errors.
- As we already set the alias `zephyr,ieee802154 = &ieee802154g`, I think
  it would be best to enable only subg by default.

Signed-off-by: Ayush Singh <ayush@beagleboard.org>