Support for indirect user-level CSRs

[christian-herber-nxp]

There have been discussions aroud using CSR access within CLIC - see riscv/riscv-fast-interrupt#349.
However, CLIC has user-level CSRs, which would be a showstopper.
@bcstrongx Does it make sense to the indirect CSR mechanism to the Unprivileged and User-Level CSRs?

[bcstrongx]

Hi Christian, I think adding user-level indirect CSRs makes sense. However, I believe the S*csrind spec is already ARC approved and ready for freeze. Let me check with ARC on how to proceed.

[bcstrongx]

The guidance I was given was that user indirect CSRs makes sense, but the proposal should be developed alongside an extension that will use it. So probably makes sense for the Fast Interrupts TG to put together a fast-track ISA extension for User Indirect CSR Access (Sucsrind), which should be pretty straight-forward since it will mirror Smcsrind and Sscsrind.