target/riscv: Add cfg property for Zvkb extension

After vector crypto spec v1.0.0-rc3 release, the Zvkb extension is
defined as a proper subset of the Zvbb extension. And both the Zvkn and
Zvks shorthand extensions replace the included Zvbb extension by Zvkb
extnesion.

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: <20231026151828.754279-4-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>