hw/riscv/riscv-iommu: implement reset protocol

hw/riscv/riscv-iommu: implement reset protocol

Add a riscv_iommu_reset() helper in the base emulation code that
implements the expected reset behavior as defined by the riscv-iommu
spec.

Devices can then use this helper in their own reset callbacks.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241106133407.604587-7-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>