resources: Add the 'arm64-ubuntu-20.04-img' resource

This references the source, not yet committed, here:
https://gem5-review.googlesource.com/c/public/gem5-resources/+/58550

Change-Id: I407598796170b1bfdb89f485bfa6305b4e8bd659
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-resources/+/59529
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
diff --git a/resources.json b/resources.json
index 0eef29b..2518336 100644
--- a/resources.json
+++ b/resources.json
@@ -134,6 +134,19 @@
         },
         {
             "type": "resource",
+            "name" : "arm64-ubuntu-20.04-img",
+            "documentation" : "A disk image containing Ubuntu 18.04 for arm64 which will run an `m5 readfile` instruction after booting. If no script file is specified an `m5 exit` instruction will be exected.",
+            "architecture" : "ARM",
+            "is_zipped" : true,
+            "md5sum" : "708cafd0591b2cdd03179b494f7bbf1f",
+            "url" : "{url_base}/images/arm/ubuntu-20-04/arm64-ubuntu-20220425.img.gz",
+            "source" : "src/arm-ubuntu",
+            "additional_metadata" : {
+                "root_partition": "1"
+            }
+        },
+        {
+            "type": "resource",
             "name" : "riscv-boot-exit-nodisk",
             "documentation" : "A simple RISCV bootloader with Linux kernel 5.10. The early userspace includes busybox and the m5 binary.",
             "architecture": "RISCV",