iommu/arm-smmu: Convert to iommu_fwspec

In the final step of preparation for full generic configuration support,
swap our fixed-size master_cfg for the generic iommu_fwspec. For the
legacy DT bindings, the driver simply gets to act as its own 'firmware'.
Farewell, arbitrary MAX_MASTER_STREAMIDS!

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>