target/riscv: Add stimecmp support

target/riscv: Add stimecmp support

stimecmp allows the supervisor mode to update stimecmp CSR directly
to program the next timer interrupt. This CSR is part of the Sstc
extension which was ratified recently.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-Id: <20220824221357.41070-3-atishp@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>