target/riscv: Add counter delegation definitions

This adds definitions for counter delegation, including the new
scountinhibit register and the mstateen.CD bit.

Signed-off-by: Kaiwen Xue <kaiwenx@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-ID: <20250110-counter_delegation-v5-6-e83d797ae294@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>