target/riscv: Expose Zvkb extension property

Signed-off-by: Max Chou <max.chou@sifive.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20231026151828.754279-6-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>