raspberrypi/4: allow pi libs to detect pi 4

raspberry-pi/4/cpu-revision.nix

@@ -0,0 +1,25 @@
+{ ... }:
+{
+  hardware.deviceTree.overlays = [
+    {
+      name = "rpi4-cpu-revision";
+      dtsText = ''
+        /dts-v1/;
+        /plugin/;
+
+        / {
+          compatible = "raspberrypi,4-model-b";
+
+          fragment@0 {
+            target-path = "/";
+            __overlay__ {
+              system {
+                linux,revision = <0x00d03114>;
+              };
+            };
+          };
+        };
+      '';
+    }
+  ];
+}

raspberry-pi/4/default.nix

@@ -3,6 +3,7 @@
 {
   imports = [
     ./audio.nix
+    ./cpu-revision.nix
     ./dwc2.nix
     ./i2c.nix
     ./modesetting.nix