CI: travis-ci: enable weekly graviton2 builds

as documented in https://blog.travis-ci.com/2020-09-11-arm-on-aws
we can build on graviton2, let us expand travis-ci matrix to that
machine type as well

.travis.yml

@@ -26,6 +26,12 @@ matrix:
     arch: arm64
     compiler: gcc
     if: type == cron
+  - os: linux
+    arch: arm64-graviton2
+    group: edge
+    virt: vm
+    compiler: gcc
+    if: type == cron
   - os: linux
     arch: s390x
     compiler: gcc