target/riscv: Add Smdbltrp CSRs handling

target/riscv: Add Smdbltrp CSRs handling

Add `ext_smdbltrp`in RISCVCPUConfig and implement MSTATUS.MDT behavior.
Also set MDT to 1 at reset according to the specification.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250110125441.3208676-7-cleger@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>