iommu/arm-smmu: Refactor master_cfg/fwspec usage

In preparation for restructuring iommu_fwspec, refactor the way we
access the arm_smmu_master_cfg private data to be less dependent on
the current layout.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20200326150841.10083-11-joro@8bytes.org