target/riscv: Add 'ext_smrnmi' in the RISCVCPUConfig

target/riscv: Add 'ext_smrnmi' in the RISCVCPUConfig

The boolean variable 'ext_smrnmi' is used to determine whether the
Smrnmi extension exists.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Signed-off-by: Tommy Wu <tommy.wu@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250106054336.1878291-2-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>