docs/specs: add riscv-iommu

docs/specs: add riscv-iommu

Add a simple guideline to use the existing RISC-V IOMMU support we just
added.

This doc will be updated once we add the riscv-iommu-sys device.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241016204038.649340-13-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>