target/riscv: Add Smrnmi cpu extension

target/riscv: Add Smrnmi cpu extension

This adds the properties for ISA extension Smrnmi.

Also, when Smrnmi is present, the firmware (e.g., OpenSBI) must set
mnstatus.NMIE to 1 before enabling any interrupts. Otherwise, all
interrupts will be disabled. Since our current OpenSBI does not
support Smrnmi yet, let's disable Smrnmi for the 'max' type CPU for
now. We can re-enable it once OpenSBI includes proper support for it.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Signed-off-by: Tommy Wu <tommy.wu@sifive.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250106054336.1878291-6-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>