hw/riscv: add riscv-iommu-pci reference device

hw/riscv: add riscv-iommu-pci reference device

The RISC-V IOMMU can be modelled as a PCIe device following the
guidelines of the RISC-V IOMMU spec, chapter 7.1, "Integrating an IOMMU
as a PCIe device".

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: <20241016204038.649340-6-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>