Include RISC-V, as requested in the last MultiXscale review

[julianmorillo]

As discussed in MultiXscale WP1+WP5 meeting: https://hackmd.io/Rc93yyJMRl64vhSTNLjfVg, reviewers complained about insufficient documentation concerning RISC-V and ARM, and about the fact that there is no overview of software available for RISC-V. This PR tries to solve/alleviate these complains.

[casparvl]

This was discussed in the support meeting

• This PR would push the available software in the existing json. Since it's a separate repository with rather different versions of things, it should be a separate json file.
• This PR only pushes it into a json file, it doesn't expose it on the website. Proposal is to expose it somewhere under https://www.eessi.io/docs/repositories/riscv.eessi.io/ (and that page needs probably an update too)

Kenneth remarked that @ocaisa will look into splitting things across multiple json's, so maybe good to align with him on who does what.