target/riscv: Add Smrnmi CSRs

target/riscv: Add Smrnmi CSRs

The Smrnmi extension adds the 'mnscratch', 'mnepc', 'mncause',
'mnstatus' CSRs.

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: <20250106054336.1878291-3-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>