target/riscv/cpu.c: add zmmul isa string

zmmul was promoted from experimental to ratified in commit 6d00ffad.
Add a riscv,isa string for it.

Fixes: 6d00ffad ("target/riscv: move zmmul out of the experimental properties")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230720132424.371132-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>