RISC-V: Add SSTC extension CSR details

This patch just introduces the required CSR fields related to the
SSTC extension.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20220722165047.519994-2-atishp@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>