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

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

The cpu->cfg.epmp extension is still experimental, but it already has a
'smepmp' riscv,isa string. Add it.

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: <20230720132424.371132-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>