Introduce vendor prefix for Rivos custom extensions

This change reserves the vendor prefix "rv." for Rivos's custom extensions.  

Signed-off-by: Philip Reames <[email protected]>

Author: preames

src/toolchain-conventions.adoc

@@ -280,6 +280,7 @@ separated by a dot, e.g., `th.vxrm`.
 |Cheriot                | ct              | https://cheriot.org/
 |Open Hardware Group    | cv              | https://www.openhwgroup.org/
 |Andes                  | nds             | https://www.andestech.com/
+|Rivos                  | rv              | https://www.rivosinc.com/
 |SiFive                 | sf              | https://www.sifive.com/
 |T-Head                 | th              | https://www.t-head.cn/
 |Tenstorrent            | tt              | https://www.tenstorrent.com/