target/riscv: rvk: add support for sha256 related instructions in zknh extension

target/riscv: rvk: add support for sha256 related instructions in zknh extension

 - add sha256sig0, sha256sig1, sha256sum0 and sha256sum1 instructions

Co-authored-by: Zewen Ye <lustrew@foxmail.com>
Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220423023510.30794-9-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>