add(asrock-a620i): new setup for testing fan

by default, system does not automatically detect fan configuration for
chassis and the cpu. quite inconvenient to setup fan speeds

modules/nixos/apps-optional.nix

@@ -1,5 +1,6 @@
 {
   imports = [
     ../../apps/qmk
+    ./asrock-a620i.nix
   ];
 }

modules/nixos/asrock-a620i.nix

@@ -0,0 +1,8 @@
+{pkgs, ...}: {
+  # fan setup
+  programs.coolercontrol.enable = true;
+  boot.kernelModules = ["nct6683"];
+  boot.extraModprobeConfig = ''
+    options nct 6683 force=1
+  '';
+}