raspberry-pi-4: add gpio-fan overlay

[matiasyocca]

Description of changes

Adds support for some PWM GPIO fans in a similar way to the official gpio-fan overlay. Tested with Argon Mini Fan.

Here's the instruction manual for the fan for reference: https://malnapc.hu/custom/malnapc/image/data/docs/RS/A700000007741984.pdf

Things done

• Tested the changes in your own NixOS Configuration
• Tested the changes end-to-end by using your fork of nixos-hardware and
  importing it via <nixos-hardware> or Flake input

[doronbehar]

@JamieMagee do you think you could test this PR?

[JamieMagee]

@JamieMagee do you think you could test this PR?

I don't own this exact fan, but comparing with the upstream gpio-fan-overlay.dts the only changes are:

1. Compatibility changed from "brcm,bcm2835" (All Raspberry Pis) to "brcm,bcm2711" (Raspberry Pi 4+)
2. GPIO pin changed from 12 to 18

I assume both of these are due to the fixed pins on this fan only working with the exact layout of the Raspberry Pi 4, and forcing pin 18. In that regard, this looks correct.

I think I'd like to try and land a generic hardware.raspberry-pi.overlays mechanism that consumes the upstream .dtbos with their __overrides__ parameters. With that, this overlay would already be supported by gpio-fan.dtbo and we'd avoid the duplication.

But I don't know how long that would take, so I'm not opposed to this PR. I'd maybe just make it clearer that the defaults are for the Argon case fan, and not something like the official Raspberry Pi 4 fan

[doronbehar]

@matiasyocca Maybe it'd be more correct to specify 12? Or 14 maybe? Here's what I saw here:

[doronbehar]

Maybe:

Suggested change

	enable = lib.mkEnableOption "support for Raspberry Pi style gpio-fan control";
	enable = lib.mkEnableOption "support for [Raspberry Pi gpio-fan](https://www.raspberrypi.com/products/raspberry-pi-4-case-fan/) control";