target/riscv: Add Smdbltrp ISA extension enable switch

target/riscv: Add Smdbltrp ISA extension enable switch

Add the switch to enable the Smdbltrp ISA extension and disable it for
the max cpu. Indeed, OpenSBI when Smdbltrp is present, M-mode double
trap is enabled by default and MSTATUS.MDT needs to be cleared to avoid
taking a double trap. OpenSBI does not currently support it so disable
it for the max cpu to avoid breaking regression tests.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250116131539.2475785-1-cleger@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>