Allow increasing `gpu_mem` for Raspberry PI

[M-Gregoire]

Hello :wave: ,

After installing NixOS and using nixos-hardware to setup my Raspberry Pi 4, the allocated GPU memory is 76M.

# vcgencmd get_mem gpu
gpu=76M

I'd like to increase it to at least 128M. I believe the standard way would have been:

  boot.loader.raspberryPi = {
    enable = true;
    firmwareConfig = ''
      gpu_mem=128
    '';
  };

But I've read in an other issue that firmwareConfig is gonna be deprecated.

I'm not quite sure what it the proper way to increase the memory allocation for the GPU.

Any help would be appreciated :)