Consider using the rust-embedded/riscv crate for CSR access

[abrestic-rivos]

Consider using the officially-supported riscv crate for CSR access rather than our local fork of Tock's riscv-csr crate. We'd need to upstream a bunch of changes to the former as it's missing most of the S/HS/VS CSRs that we care about in Salus.

cc @FawazTirmizi

[FawazTirmizi]

There is a fork of the riscv crate in our organization as well (here). We (firmware) were planning on adding S-mode CSRs ourselves at some point anyways so I'm happy to take care of that. For hypervisor-related stuff, adding the new registers should also be a straightforward task, but likely not as high of a priority for us.

It may be possible to switch the crate to using the interfaces from riscv-csr if that'd be preferable, but I think we should see what upstream thinks of that before going with that option.