boards: beaglev_fire: Force 'medany' code model

The 'beaglev_fire' board has its SRAM located at 0x1000000000, and this
causes Zephyr to default to the `large` code model.

This commit forces the board to use `medany` code model because the `large`
code model is not supported by GCC 12.2 included in the current Zephyr SDK
0.17.2.

Revert this when Zephyr SDK 0.18.0 with GCC 14.3 is mainlined.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>