Implement the Smcsrind/Sscsrind extension v1.0

[emanueleparisi]

Implement the Smcsrind/Sscsrind standard extension to CVA6. This extension confers two benefits:

• It provides a means to access an array of registers via CSRs without requiring allocation of large chunks of the limited CSR address space.
• It enables software to access each of an array of registers by index, without requiring a switch statement with a case for each register.

The current implementation makes the following changes:

1. Add the appropriate definitions for the CSR_*ISELECT and CSR_*IREG* inside the riscv_pkg.sv.
2. Defines two handy types (csrind_isel_t and csrind_ireg_t) in ariane_pkg.sv to represent the selection register and the alias registers.
3. Add three CSRs (miselect, siselect, and vsiselect) in csr_regfile.sv.
4. Modify the CSR read/write logic in csr_regfile.sv to read and write the added CSRs.

[emanueleparisi]

Additionally, please find the smcsrind.txt file containing a C source usable to test the implemented extension in cheshire (main branch).