target/riscv: Add smstateen support

target/riscv: Add smstateen support

Smstateen extension specifies a mechanism to close
the potential covert channels that could cause security issues.

This patch adds the CSRs defined in the specification and
the corresponding predicates and read/write functions.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221016124726.102129-2-mchitale@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>