Fix only 4GiB of memory being available

This might have been caused by the switch to
generic-extlinux-compatible, but u-boot seems to be honouring the FDT
option.

modules/boot/8gb-patch.dts

@@ -0,0 +1,11 @@
+/dts-v1/;
+/plugin/;
+/ {
+	compatible = "starfive,jh7110";
+	fragment@0 {
+		target-path = "/memory@40000000";
+		__overlay__ {
+			reg = <0x0 0x40000000 0x2 0x0>;
+		};
+	};
+};

modules/boot/default.nix

@@ -50,6 +50,10 @@
   hardware.deviceTree.name = "starfive/jh7110-starfive-visionfive-2-v1.3b.dtb";
 
   hardware.deviceTree.overlays = [
+    {
+      name = "8gb-patch";
+      dtsFile = ./8gb-patch.dts;
+    }
     # https://github.com/starfive-tech/linux/pull/99
     {
       name = "qspi-patch";