hw/riscv/riscv-iommu.c: add riscv_iommu_instance_init()

hw/riscv/riscv-iommu.c: add riscv_iommu_instance_init()

Move all the static initializion of the device to an init() function,
leaving only the dynamic initialization to be done during realize.

With this change s->cap is initialized with RISCV_IOMMU_CAP_DBG during
init(), and realize() will increment s->cap with the extra caps.

This will allow callers to add IOMMU capabilities before the
realization.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241106133407.604587-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>