hw/riscv/virt: Add IOMMU as platform device if the option is set

hw/riscv/virt: Add IOMMU as platform device if the option is set

Add a new machine option called 'iommu-sys' that enables a
riscv-iommu-sys platform device for the 'virt' machine. The option is
default 'off'.

The device will use IRQs 36 to 39.

We will not support both riscv-iommu-sys and riscv-iommu-pci devices in
the same board in this first implementation. If a riscv-iommu-pci device
is added in the command line we will disable the riscv-iommu-sys device.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241106133407.604587-5-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>