target/riscv/debug.c: use wp size = 4 for 32-bit CPUs

target/riscv/debug.c: use wp size = 4 for 32-bit CPUs

The mcontrol select bit (19) is always zero, meaning our triggers will
always match virtual addresses. In this condition, if the user does not
specify a size for the trigger, the access size defaults to XLEN.

At this moment we're using def_size = 8 regardless of CPU XLEN. Use
def_size = 4 in case we're running 32 bits.

Fixes: 95799e36c1 ("target/riscv: Add initial support for the Sdtrig extension")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250121170626.1992570-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>