[WATCHLISTS] Add riscv watch

Change-Id: I6e4dc69d6f22d3108ae74552b72bcafc0be3db64
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3793476
Commit-Queue: Yahan Lu <yahan@iscas.ac.cn>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82092}

WATCHLISTS

@@ -129,6 +129,7 @@
     'arm': [
       'v8-mips-ports@googlegroups.com',
       'v8-ppc-ports@googlegroups.com',
+      'v8-risc-v-ports@googlegroups.com',
     ],
     'merges': [
       # Only enabled on branches created with tools/release/create_release.py