hw/riscv/riscv-iommu: add DBG support

hw/riscv/riscv-iommu: add DBG support

DBG support adds three additional registers: tr_req_iova, tr_req_ctl and
tr_response.

The DBG cap is always enabled. No on/off toggle is provided for it.

Signed-off-by: Tomasz Jeznach <tjeznach@rivosinc.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241016204038.649340-11-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>